Fix --mtu-disc maybe|yes on Linux.

Fix --mtu-disc maybe|yes on Linux.

--mtu-disc (on Linux) needs two components to work:
   - setsockopt() with IP_MTU_DISCOVER or IPV6_MTU_DISCOVER
   - "extended error reporting" (setsockopt(IP_RECVERR) and
     then via mtu.c/format_extended_socket_error()) to react on
     "packet too big" errors on sendto() / sendmsg()

Some configure.ac reorganization broke detection of <linux/errqueue.h>
and "struct sock_extended_err".  Re-add <linux/errqueue.h> to configure.ac,
remove all the other conditionals in syshead.h, and remove the
"struct sock_extended_err" check completely (assumption: if errqueue.h
exists, it contains what we need).

Thus, the "non-helpful" socket error message turns into:

2022-02-22 12:31:42 write UDPv4 [EMSGSIZE Path-MTU=800]: Message too long (fd=3,code=90)
2022-02-22 12:31:42 Note adjusting 'mssfix 1400 mtu' to 'mssfix 800 mtu' according to path MTU discovery
2022-02-22 12:31:42 Note adjusting 'fragment 1400 mtu' to 'fragment 800 mtu' according to path MTU discovery

... while at it, fix extra space in first part of these messages, and
print o->ce.fragment for the "fragment" message...

v2: assume that "if it's linux, and has these two headers, everything
else will be there as well" and get rid of most of the #ifdef checks

Trac: #1452

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20220222113832.13383-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23863.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>