Reset terminal styles

Reset terminal styles

This adds a function that can be used to reset terminal styles,
regardless of what style the low-level output routines currently think
is applied.

This is used to make "echo" and "printf" work properly when emitting
ANSI terminal escapes -- now gdb will reset the style at the end of
the command.

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

	* utils.h (reset_terminal_style): Declare.
	* utils.c (can_emit_style_escape): New function.
	(set_output_style): Use it.
	(reset_terminal_style): New function.
	* printcmd.c (printf_command): Call reset_terminal_style.
	* cli/cli-cmds.c (echo_command): Call reset_terminal_style.