Improve error reporting from AF_UNIX tun/tap support

Improve error reporting from AF_UNIX tun/tap support

When having a non-existent lwipovpn binary or similar problems, the
error reporting would often only report read error that were harder to
identify the real problem.  Add the openvpn_waitpid_check method
that checks for error conditions and reports a better message in cases
of problems.

Change-Id: I81cbecd19018290d85c6c77fba7769f040d66233
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20250201122006.32098-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg30782.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>