Do not rely on hardcoded path to systemd-ask-password.

Relying on hardcoded /bin/systemd-ask-password path breaks systemd that
install systemd-ask-password in /usr/bin. Since both paths are supposed
to be in ${PATH} and popen() passes the command to shell, just pass
'systemd-ask-password' and let the shell find it.

Fixes: https://bugzilla.samba.org/show_bug.cgi?id=10054
Signed-off-by: Michał Górny <mgorny@gentoo.org>