Remove stray comma for XRandR detection

Fix a stray comma interpolating into the configure script as an action to
run, causing it to print harmless errors.

Noticed by Hendrik Jaeger.
Add strtonum

This adds strtonum() from OpenBSD.
Deprecate atoi() in favour of strtonum()

Rather than rely on atoi()s side-effects, instead switch to using strtonum()
which is more security-conscious (and less broken) than atoi() and friends,
and hence allows for better bounds-checking, etc.