Allow remote debugging over a Unix local domain socket.

Allow remote debugging over a Unix local domain socket.

Extend the "target remote" and "target extended-remote" commands
such that if the filename provided is a Unix local domain (AF_UNIX)
socket, then it'll be treated as such, instead of trying to open
it as if it were a character device.

gdb/ChangeLog:
	* NEWS: Mention changed commands.
	* ser-uds.c: New file.
	* configure.ac (SER_HARDWIRE): Add ser-uds.o.
	* configure: Regenerate.
	* Makefile.in: Add new file.
	* serial.c (serial_open): Check if filename is a socket
	  and lookup the appropriate interface accordingly.

gdb/doc/ChangeLog:
	* gdb.texinfo (Remote Connection Commands): Describe
	  the changes to target remote and target extended-remote
	  relating to Unix domain sockets.