core: add options to customize commands on system signals, quit by default on...

core: add options to customize commands on system signals, quit by default on SIGHUP when not running headless (closes #1595)

New options to customize behavior on signals received, with the default
behavior:

- weechat.signal.sighup: quit in normal mode, reload config in headless
- weechat.signal.sigquit: quit
- weechat.signal.sigterm: quit
- weechat.signal.sigusr1: no command executed by default
- weechat.signal.sigusr2: no command executed by default

The signals SIGUSR1 and SIGUSR2 are introduced by this commit, so it's now
possible to run commands when they are received.

The SIGHUP signal makes now WeeChat quit, it was the behavior before version
2.9 of WeeChat (see commit de1e61f7).