Preparing for better signal handling: some code refactoring

Preparing for better signal handling: some code refactoring

- Do not directly update signal_received: always use register_signal()
  throw_signal() or signal_reset().
  To facilitate this, register_signal() now takes c->sig as an argument
  instead of the context c itself, and sig_info struct is passed-in to
  functions that need to set a signal.

- openvpn_getaddrinfo() is updated in a following commit as it
  could benefit from some logic changes that we may or may not want
  to do.

No functional changes.

TODO:
(i)   update signal handling in openvpn_getaddrinfo
(ii)  enforce signal priority
(iii) fix signal handling on Windows
for 2.7?
(iv)  replace system-V signal with POSIX sigaction

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20230101215109.1521549-2-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25874.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 05715485b45816e18b52ffb9b47ca22a55abb334)