Add FreeBSD-specific PAM configuration

The "login" PAM configuration means somathing entirely different on
FreeBSD than on Linux: if you try to authenticate as the calling user,
it OKs the request without prompting for password. The "passwd" config
implements the desired functionality, therefore it should be used by
swaylock.

swaylock/meson.build

@@ -24,7 +24,16 @@ executable(
 	install: true
 )
 
+if is_freebsd
 	install_data(
-	'pam/swaylock',
-	install_dir: sysconfdir + '/pam.d/'
+		'pam/swaylock.freebsd',
+		install_dir: sysconfdir + '/pam.d/',
+		rename: 'swaylock'
 	)
+else
+	install_data(
+		'pam/swaylock.linux',
+		install_dir: sysconfdir + '/pam.d/',
+		rename: 'swaylock'
+	)
+endif

swaylock/pam/swaylock.freebsd

@@ -0,0 +1,6 @@
+#
+# PAM configuration file for the swaylock screen locker. By default, it includes
+# the 'passwd' configuration file (see /etc/pam.d/passwd)
+#
+
+auth include passwd