Improve signal handling using POSIX sigaction

Improve signal handling using POSIX sigaction

Currently we use the old signal API which follows system-V or
BSD semantics depending on the platform and/or feature-set macros.
Further, signal has many weaknesses which makes proper masking
(deferring) of signals during update not possible.

Improve this:

- Use sigaction to properly mask signals when modifying.

Notes:

Updating signal_reset() is handled in a follow up patch

SIG_SOURCE_CONNECTION_FAILED is retained in a hackish way. This value
has the same meaning as SIG_SOURCE_SOFT everywhere except where the
signal is printed. Looks cosmetic --- could be eliminated?

In pre_init_signal_catch() we ignore some unix signals, but the same
signals from management are not ignored though both are treated as "HARD"
signals.  For example, during auth-user-pass query, "kill -SIGUSR1 <pid>"
will be ignored, but "signal SIGUSR1" from management interface will
cause M_FATAL and exit.  This is the current behaviour, but could be improved?

This patch was originally submitted as 5/5 of the signals series. Now this
is 1/2 of a new series with signal_reset changes moved to 2/2

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20230128215901.2207208-1-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26087.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>