agent: kill pinentry by SIGINT, fixing a bug to be killed by SIGINT.

* agent/call-pinentry.c (atfork_cb): Reset signal mask and signal
handler for child process.
(agent_popup_message_stop): Send SIGINT (was: SIGKILL).

pinentry-curses should be killed by SIGINT, so that it can reset
terminal settings, but it didn't work.