ps/output.c: Protect outbuf in various pr_*() functions.

ps/output.c: Protect outbuf in various pr_*() functions.

pr_bsdstart(): Replace "strcpy(outbuf," with "snprintf(outbuf, COLWID,"
(which is used in all surrounding functions). (side note: the fact that
many pr_*() functions simply return "snprintf(outbuf, COLWID," justifies
the "amount" checks added to show_one_proc() by the "ps/output.c:
Replace strcpy() with snprintf() in show_one_proc()." patch)

pr_stime(): Check the return value of strftime() (in case of an error,
"the contents of the array are undefined").

help_pr_sig(): Handle the "len < 8" case, otherwise "sig+len-8" may
point outside the sig string.

pr_context(): Handle the empty string case, or else "outbuf[len-1]"
points outside outbuf.