MDEV-15655: Add Linux abstract socket support

The functionality of the socket system variable is extended
here such that a preciding '@' indicates that the socket
will be an abstract socket. Thie socket name wil be
the remainder of the name after the '@'. This is consistent
with the approached used by systemd in socket activation.

Thanks to Sergey Vojtovich:

On OS X sockaddr_un is defined as:

struct sockaddr_un
{
  u_char sun_len;
  u_char sun_family;
  char  sun_path[104];
};

There is a comment in man 7 unix (on linux):

"
On Linux, the above offsetof() expression equates to the same value as sizeof(sa_family_t),
but some other implementations include other fields before sun_path, so the offsetof()
expression more portably describes the size of the address structure.
"

As such, use the offsetof for Linux and use the previous sizeof(UNIXaddr)
for non-unix platforms as that's what worked before and they don't
support abstract sockets so there's no compatibility problem..

strace -fe trace=networking mysqld --skip-networking --socket @abc ...
...
[pid 10578] socket(AF_UNIX, SOCK_STREAM|SOCK_CLOEXEC, 0) = 22
[pid 10578] setsockopt(22, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
[pid 10578] bind(22, {sa_family=AF_UNIX, sun_path=@"abc"}, 6) = 0
[pid 10578] listen(22, 80)              = 0
...
Version: '10.3.6-MariaDB-log'  socket: '@abc'  port: 0  Source distribution

$ lsof -p 10578
mysqld  10578  dan   22u  unix 0x00000000087e688c       0t0 4787815 @abc type=STREAM

client/mysqltest.cc

@@ -6010,9 +6010,10 @@ void do_connect(struct st_command *command)
   {
     /*
       If the socket is specified just as a name without path
+      or an abstract socket indicator ('@'), then
       append tmpdir in front
     */
-    if (*ds_sock.str != FN_LIBCHAR)
+    if (*ds_sock.str != FN_LIBCHAR && *ds_sock.str != '@')
     {
       char buff[FN_REFLEN];
       fn_format(buff, ds_sock.str, TMPDIR, "", 0);