Always return 1 when execvp() fails.

[emanuele6]

bspwm uses execvp() to replace itself with a new instance in response to "wm -r", and exits with the return value of that execvp() call if it fails.

execvp() only returns when it fails; and, when it returns, it always returns -1 which is not a valid exit code (it is treated as 255: &0xff).

The return value of execvp() is not really meaningful; let's just exit with `1' if execvp() fails.