gdbstub: unlink the unix socket before bind()

This is another variant of doing what v9.2.0-1561-gfccb744f41c6
"gdbstub: Try unlinking the unix socket before binding" did, but
that commit introduced dependency on qemu-sockets.c which is
more problematic for statically-linked qemu-user binaries.

Reviewed-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

gdbstub/user.c

@@ -337,6 +337,7 @@ static int gdbserver_open_socket(const char *path)
 
     sockaddr.sun_family = AF_UNIX;
     pstrcpy(sockaddr.sun_path, sizeof(sockaddr.sun_path) - 1, path);
+    unlink(sockaddr.sun_path);
     ret = bind(fd, (struct sockaddr *)&sockaddr, sizeof(sockaddr));
     if (ret < 0) {
         perror("bind socket");