Stop using $USER for shadow entries

Stop using $USER for shadow entries

This was extremely bad practice, effectively making the program behave
different depending on which architecture you are running it on.

OpenBSD offers getpwuid_shadow, but there is no getspuid for getspnam,
so we resort to using the pw_name entry in the struct passwd we filled
earlier.

This prevents slock from crashing when $USER is empty (easy to do). If
you want to run slock as a different user, don't use

	$ USER="tom" slock

but doas or sudo which were designed for this purpose.