setup: print paths to the settings being setup

This commit introduces setting path printing to interactive
setup, making it much more clear what on earth you're setting up
in more complex scenarios.

e.g. an interactive setup of module=compose is now:

```
$ src/rototiller --module=compose

/module/compose/layers:
 Comma-separated list of module layers, in draw-order [drizzle,stars,spiro,plato]:

/module/compose/layers/[0]/drizzle/viscosity:
 Puddle viscosity:
  0: .005
  1:  .01
  2:  .03
  3:  .05
 Enter a value 0-3 [1 (.01)]:

/module/compose/layers/[0]/drizzle/style:
 Overlay style:
  0: mask
  1:  map
 Enter a value 0-1 [0 (mask)]:

/module/compose/layers/[1]/stars/rot_adj:
 Rotation rate:
  0:     .0
  1: .00001
  2: .00003
  3:  .0001
  4:  .0003
  5:   .001
 Enter a value 0-5 [2 (.00003)]:

/module/compose/layers/[3]/plato/orbit_rate:
 Orbit rate and direction:
  0:   -1
  1: -.75
  2:  -.5
  3: -.25
  4:   .1
  5:    0
  6:   .1
  7:  .25
  8:   .5
  9:  .75
 10:    1
 Enter a value 0-10 [7 (.25)]:

/module/compose/layers/[3]/plato/spin_rate:
 Spin rate and direction:
  0:   -1
  1: -.75
  2:  -.5
  3: -.25
  4:   .1
  5:    0
  6:   .1
  7:  .25
  8:   .5
  9:  .75
 10:    1
 Enter a value 0-10 [9 (.75)]:

/module/compose/texture:
 Module to use for source texture, "none" to disable:
  0:     none
  1:   blinds
  2: checkers
  3:  drizzle
  4:    julia
  5:    moire
  6:   plasma
  7:     roto
  8:    stars
  9:   submit
 10:     swab
 11:  voronoi
 Enter a value 0-11 [0 (none)]: 1

/module/compose/texture/blinds/orientation:
 Blinds orientation:
  0: horizontal
  1:   vertical
 Enter a value 0-1 [0 (horizontal)]:

/module/compose/texture/blinds/count:
 Blinds count:
  0:  2
  1:  4
  2:  8
  3: 12
  4: 16
  5: 24
  6: 32
 Enter a value 0-6 [4 (16)]: 3

/video:
 Video backend:
  0: drm
  1: mem
  2: sdl
 Enter a value 0-2 [2 (sdl)]:

/video/sdl/fullscreen:
 SDL fullscreen mode:
  0: off
  1:  on
 Enter a value 0-1 [0 (off)]:

/video/sdl/size:
 SDL window size [640x480]:

Configured settings as flags:
  --seed=0x6471f827 '--module=compose,layers=drizzle\\\,viscosity\\\=.01\\\,style\\\=mask\,stars\\\,rot_adj\\\=.00003\,spiro\,plato\\\,orbit_rate\\\=.25\\\,spin_rate\\\=.75,texture=blinds\,orientation\=horizontal\,count\=12' '--video=sdl,fullscreen=off,size=640x480'

Press enter to continue, add --go to skip this step...
```

Previously it would be like so:
$ src/rototiller --module=compose

Comma-separated list of module layers, in draw-order [drizzle,stars,spiro,plato]:

Puddle viscosity:
 0: .005
 1:  .01
 2:  .03
 3:  .05
Enter a value 0-3 [1 (.01)]:

Overlay style:
 0: mask
 1:  map
Enter a value 0-1 [0 (mask)]:

Rotation rate:
 0:     .0
 1: .00001
 2: .00003
 3:  .0001
 4:  .0003
 5:   .001
Enter a value 0-5 [2 (.00003)]:

Orbit rate and direction:
 0:   -1
 1: -.75
 2:  -.5
 3: -.25
 4:   .1
 5:    0
 6:   .1
 7:  .25
 8:   .5
 9:  .75
10:    1
Enter a value 0-10 [7 (.25)]:

Spin rate and direction:
 0:   -1
 1: -.75
 2:  -.5
 3: -.25
 4:   .1
 5:    0
 6:   .1
 7:  .25
 8:   .5
 9:  .75
10:    1
Enter a value 0-10 [9 (.75)]:

Module to use for source texture, "none" to disable:
 0:     none
 1:   blinds
 2: checkers
 3:  drizzle
 4:    julia
 5:    moire
 6:   plasma
 7:     roto
 8:    stars
 9:   submit
10:     swab
11:  voronoi
Enter a value 0-11 [0 (none)]: 1

Blinds orientation:
 0: horizontal
 1:   vertical
Enter a value 0-1 [0 (horizontal)]:

Blinds count:
 0:  2
 1:  4
 2:  8
 3: 12
 4: 16
 5: 24
 6: 32
Enter a value 0-6 [4 (16)]: 3

Video backend:
 0: drm
 1: mem
 2: sdl
Enter a value 0-2 [2 (sdl)]:

SDL fullscreen mode:
 0: off
 1:  on
Enter a value 0-1 [0 (off)]:

SDL window size [640x480]:

Configured settings as flags:
  --seed=0x6471f827 '--module=compose,layers=drizzle\\\,viscosity\\\=.01\\\,style\\\=mask\,stars\\\,rot_adj\\\=.00003\,spiro\,plato\\\,orbit_rate\\\=.25\\\,spin_rate\\\=.75,texture=blinds\,orientation\=horizontal\,count\=12' '--video=sdl,fullscreen=off,size=640x480'

Press enter to continue, add --go to skip this step...
```

Which gets rather confusing, especially if you go even deeper
with nested module situations like
compose/layers/[0]/checkers/fill_module/compose/layers/[0]/drizzle

You just lose track of which module instance's settings you're
actually setting up

This is just the first step of making use of these paths.  In the
future the module context paths will be derived from the settings
paths so they're more reliably distinct and descriptive, along
with consistent with paths in the settings namespace.  That
becomes important when module contexts start getting registered
on the stream for use in other modules.

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