Replace pidof usage with pgrep

The patch changes the logic to use pgrep, now included as a standard tool in most BSD and linux systems, in place of using pidof.

The change also simplifies logic a little.

I have performed some testing using the "switch user" functionality in the screensaver and everything works as expected.

This has been requested by a FreeBSD user [1]

[1] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=290766

---

Fixes: #38 (closed)