Fix bug that program_invocation_name is NULL when backgrounding

Fix bug that program_invocation_name is NULL when backgrounding

This is a weird issue. When elogind is started from a tty, everything
works just fine. But since adding the fixes to build elogind against
the musl libc, which does not provide program_invocation_name and
program_invocation_short_name, glibc does not set them when elogind
is background. Neither when done manually, nor when doing it using a
system init tool like openrc /sbin/start-stop-daemon.

The "workaround" is to allow elogind_set_program_name() to (re-)set
both globals if they are either NULL, or not set to the correct
value.

This should work fine with both glibc and musl-libc.