/proc/self/exe doesn't exist, but $C already gives us the path to
the binary. /dev/full doesn't have any equivalent afaik, so skip
that test if /dev/full is missing.
"cat /proc/self/exe && cmp /proc/self/exe" won't see the same file if cat
and cmp aren't both in a toybox multiplexer binary, so snapshot and pick one.
