charts: show ??s when proc_stat->start is unset

When libvmon fails to successfully sample proc_stat, it will
leave this value as 0, which isn't really otherwise a normal
process start value.

Handle this by producing "??s" for the Wall time normally derived
from (sys_stat->boottime - proc_stat->start), to prevent
producing an incorrect Wall time equal to sys_stat->boottime.

There should probably be a more robust means of communicating
these libvmon sampling failures to vwm/vmon, but I've thus far
been resisting adding something like an errno to every sample
store, or worse every sample store's datum.  It's kind of
non-trivial to do without bloating the sample stores, especially
since the stores consolidate multiple proc files under a single
store/want.  Having a single errno in the store would prevent
letting the valid portions of the store be usable while ignoring
the errored portions.  Perhaps just a per-store errno with a
bitfield to indicate which subset are errored would suffice...

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