Move "watchdog" to remote.c

Move "watchdog" to remote.c

The "watchdog" global is only used in remote.c, so this patch moves it
there and makes it static.

gdb/ChangeLog
2019-05-08  Tom Tromey  <tromey@adacore.com>

	* maint.c (_initialize_maint_cmds): Move initialization code to
	remote.c.
	(watchdog, show_watchdog): Move to remote.c.
	* remote.c (watchdog, show_watchdog): Move from maint.c.  Make
	"watchdog" static.
	(_initialize_remote): Move initialization code from maint.c.
	* defs.h (watchdog): Don't declare.