emacsclient: fix some races on POSIX systems

emacsclient: fix some races on POSIX systems

Fix some longstanding race conditions due to emacsclient’s use of
‘signal’ instead of ‘sigaction’ and its use of nested signal
handlers.  These races could cause premature exit or incorrect
commands sent to Emacs.
* lib-src/emacsclient.c (signal) [!WINDOWSNT]: Do not undef.
(emacs_socket): Remove this static variable.  It is now a parameter.
(send_to_emacs): Do not exit merely because ‘send’ was interrupted.
Instead, act on the signal if possible, and then retry the ‘send’.
(pass_signal_to_emacs): Remove; now done by act_on_signals.
(reinstall_handler_if_needed, handle_sigttou, handle_sigwinch)
(install_handler): New functions.
(got_sigcont, got_sigtstp, got_sigttou, got_sigwinch):
New globals, used for more-portable signal handling.
(handle_sigcont, handle_sigtstp): Just set the static var; other
actions are now done later by act_on_signals.
(install_handler): New function that arranges for signals to
never be reset to default, on modern POSIX platforms.
This fixes some races.
(act_on_signals): New function.  When acting on SIGCONT,
don’t bother calling getpgrp if tcgetpgrp fails.
(start_daemon_and_retry_set_socket): Return the socket
rather than setting a global variable.  All uses changed.
(flush_stdout): New function that acts on signals received while
flushing.
(main): Use it.  emacs_socket is now a local var.
Act on signals received during recv.