pam: add call to pam_umask

Setting umask for user sessions via UMASK setting in /etc/login.defs is
a well-known feature. Let's make sure that user manager also runs with
this umask value.

Follow-up for 5e37d19.

(cherry picked from commit 159f1b78576ce91c3932f4867f07361a530875d3)

Resolves: RHEL-28048

src/login/systemd-user.m4

@@ -10,4 +10,5 @@ session required pam_selinux.so nottys open
 session required pam_loginuid.so
 session optional pam_keyinit.so force revoke
 session required pam_namespace.so
+session optional pam_umask.so silent
 session optional pam_systemd.so