Add more generic checks for xrandr and freebsd support

[l2ol33rt]

Freebsd puts the xrandr binary in /usr/local/bin/xrandr. Instead I just made the check more generic.

Appreciate the project, comes in handy for my freebsd/i3 laptop install!

Let me know if you need me to consider anything else.

~Rob

[Ventto]

Thanks for your contribution. Indeed.
Bash being the shell/hashbang, I'll consistently use hash.
In a near future, I'll turn mons into a POSIX script. So I recommand using command -v instead.

[l2ol33rt]

@Ventto good point! Actually didnt even realize it was using bash. Went ahead and made the change to using command -v and seems to be working as expected. Also spun out an issue to track the POSIX script migration in the future.

[Ventto]

@l2ol33rt, thanks for changes.
I will soon stride along the POSIX way.