MINOR: init: emit warning when -sf/-sd cannot parse argument

MINOR: init: emit warning when -sf/-sd cannot parse argument

Previously, -sf and -sd command line parsing used atol which cannot
detect errors.  I had a problem where I was doing -sf "$pid1 $pid2 $pid"
and it was sending the gracefully terminate signal only to the first pid.
The change uses strtol and checks endptr and errno to see if the parsing
worked.  It will exit when the pid list is not parsed.

[wt: this should be backported to 1.8]