[Toybox] [PATCH] ps: exit with exit status 1 if no processes shown.

[enh]

This matches procps ps, and at least one LTP test relies on it. The
behavior is specifically "nothing matches", not "one PID didn't match":
assuming no process 666, `ps 666` exits with 1, but `ps 1 666` exits
with 0.
---
 toys/posix/ps.c | 1 +
 1 file changed, 1 insertion(+)
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