tty-askpw-agent: react to SIGTERM while waiting for console

I noticed that systemd-tty-password-agent would time out when asked to
stop via SIGTERM, and eventually be killed, under some circumstances.
It took me a while but i figured out what was going on:

systemd-ask-pw-agent blocks SIGTERM because it wants async notifications
on SIGTERM via signalfd() to listen on. That mostly works great: except
for one case: if we actually get a pw query request, and hence need to
acquire the terminal: we issue open_terminal() in that case, but if the
terminal is used otherwsie we'll hang, and because SIGTERM is blocked
we'll hang and cannot exit cleanly.

Address that: optionally, in acquire_terminal() look for SIGTERM by
unblcking the signal mask via ppoll() while we wait.

src/basic/io-util.c

@@ -188,7 +188,7 @@ int pipe_eof(int fd) {
         return !!(r & POLLHUP);
 }
 
-int ppoll_usec(struct pollfd *fds, size_t nfds, usec_t timeout) {
+int ppoll_usec_full(struct pollfd *fds, size_t nfds, usec_t timeout, const sigset_t *ss) {
         int r;
 
         assert(fds || nfds == 0);
@@ -211,7 +211,7 @@ int ppoll_usec(struct pollfd *fds, size_t nfds, usec_t timeout) {
         if (nfds == 0)
                 return 0;
 
-        r = ppoll(fds, nfds, timeout == USEC_INFINITY ? NULL : TIMESPEC_STORE(timeout), NULL);
+        r = ppoll(fds, nfds, timeout == USEC_INFINITY ? NULL : TIMESPEC_STORE(timeout), ss);
         if (r < 0)
                 return -errno;
         if (r == 0)