Add a "context" argument to add_setshow_enum_cmd

Add a "context" argument to add_setshow_enum_cmd

This adds a "context" argument to add_setshow_enum_cmd.  Now
add_setshow_enum_cmd will call set_cmd_context on both of the new
commands.  This is used in a later patch.

gdb/ChangeLog
2018-12-28  Tom Tromey  <tom@tromey.com>

	* command.h (add_setshow_enum_cmd): Add "context" argument.
	* cli/cli-decode.c (add_setshow_enum_cmd): Add "context"
	argument.  Call set_cmd_context.