gdbserver: Add debug-file option

gdbserver: Add debug-file option

Add command line option to send all debug output to a given file.
Always default back to stderr.

Add matching monitor command. Add documentation.

gdb/doc/ChangeLog:

	* gdb.texinfo
	(Other Command-Line Arguments for gdbserver): Add debug-file
	option.
	(Monitor Commands for gdbserver): Likewise.
	(gdbserver man): Likewise.

gdb/gdbserver/ChangeLog:

	* debug.c (debug_set_output): New function.
	(debug_vprintf): Send output to debug_file.
	(debug_flush): Likewise.
	* debug.h (debug_set_output): New declaration.
	* server.c (handle_monitor_command): Add debug-file option.
	(captured_main): Likewise.