check_procs: filter out self by /proc/pid/exe

Make check_procs filter out itself in the process list by comparing the
file pointed to by /proc/pid/exe. On platforms where this is not
available or when check_procs is passed the -T flag, the old behaviour
(check whether PID equals getpid()) is retained.

This fixes some false alarms when e.g. Nagios has, for whatever reasons,
some backlog of checks to run and check_procs with -a is called more
than once in a short time, matching its sister process.
check_procs: fix typo

Fix a typo in previous commit where st_ was missing in the mock
struct_stat_t, causing check_procs.c to not compile without sys/stat.h.
check_procs: Ignore ENOENT when checking for myself

Previously, when a process exited between the call to /bin/ps and
stat("/proc/his/exe") was exiting it was not considered as possible
instance of check_procs. This commit makes check_procs ignore all
processes where /proc/pid/exe does not exist.
check_procs: Get rid of HAVE_PROC_PID_EXE

Don't check for /proc/pid/exe at compile time, it could be gone in a
build chroot or the like. Checking at runtime (once) is sufficient.
check_procs: Assume we have stat()

stat() is in gnulib anyway, so we can safely depend on it.
check_procs: clarify debug message
NEWS: Mention the new check_procs behavior

Closes #60.