execute: make sure Type=exec and PAMName= work together

If PAMName= is used we'll spawn a PAM session for the service, and leave
a process around that closes the PAM session eventually. That process
must close the "exec_fd" that we use to implement Type=exec. After all
the logic relies on the fact that execve() will implicitly close the
exec_fd, and the EOF seen on it is hence indication for the service
manager that execve() has worked. But if we keep an fd open in the PAM
service process, then this is not going to work.

Hence close the fd explicitly so that it definitely doesn't stay pinned
in the child.

src/core/exec-invoke.c

@@ -1107,7 +1107,8 @@ static int setup_pam(
                 gid_t gid,
                 const char *tty,
                 char ***env, /* updated on success */
-                const int fds[], size_t n_fds) {
+                const int fds[], size_t n_fds,
+                int exec_fd) {
 
 #if HAVE_PAM
 
@@ -1210,6 +1211,11 @@ static int setup_pam(
                  * those fds are open here that have been opened by PAM. */
                 (void) close_many(fds, n_fds);
 
+                /* Also close the 'exec_fd' in the child, since the service manager waits for the EOF induced
+                 * by the execve() to wait for completion, and if we'd keep the fd open here in the child
+                 * we'd never signal completion. */
+                exec_fd = safe_close(exec_fd);
+
                 /* Drop privileges - we don't need any to pam_close_session and this will make
                  * PR_SET_PDEATHSIG work in most cases.  If this fails, ignore the error - but expect sd-pam
                  * threads to fail to exit normally */
@@ -4631,7 +4637,7 @@ int exec_invoke(
                  * wins here. (See above.) */
 
                 /* All fds passed in the fds array will be closed in the pam child process. */
-                r = setup_pam(context->pam_name, username, uid, gid, context->tty_path, &accum_env, params->fds, n_fds);
+                r = setup_pam(context->pam_name, username, uid, gid, context->tty_path, &accum_env, params->fds, n_fds, params->exec_fd);
                 if (r < 0) {
                         *exit_status = EXIT_PAM;
                         return log_exec_error_errno(context, params, r, "Failed to set up PAM session: %m");