Have 'thread|frame apply' style their output.

'thread|frame apply CMD' launches CMD so that CMD output goes to a string_file.
This patch ensures that string_file for such CMD output contains
style escape sequences that 'thread|frame apply' will later on
output on the real terminal, so as to have CMD output properly styled.

The idea is to have the class ui_file having overridable methods
to indicate that the output to this ui_file should be done using
'terminal' behaviour such as styling.
Then these methods are overriden in string_file so that a specially
constructed string_file will get output with style escape sequences.

After this patch, the output of CMD by thread|frame apply CMD is styled
similarly as when CMD is launched directly.
Note that string_file (term_out true) could also support wrapping,
but this is not done (yet?).

Tested on debian/amd64.

gdb/ChangeLog
2019-04-27  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	Support style in 'frame|thread apply'

	* gdbcmd.h (execute_command_to_string): New term_out parameter.
	* record.c (record_start, record_stop): Update callers of
	execute_command_to_string with false.
	* ui-file.h (class ui_file): New term_out and can_emit_style_escape
	methods.
	(class string_file): New constructor with term_out parameter.
	Override methods term_out and can_emit_style_escape.  New member
	term_out.
	(class stdio_file): Override can_emit_style_escape.
	(class tee_file): Override term_out and can_emit_style_escape.
	* utils.h (can_emit_style_escape): Remove.
	* utils.c (can_emit_style_escape): Likewise.
	Update all callers of can_emit_style_escape (SOMESTREAM) to
	SOMESTREAM->can_emit_style_escape.
	* source-cache.c (source_cache::get_source_lines): Likewise.
	* stack.c (frame_apply_command_count): Call execute_command_to_string
	passing the term_out characteristic of the current gdb_stdout.
	* thread.c (thr_try_catch_cmd): Likewise.
	* top.c (execute_command_to_string): pass term_out parameter
	to construct the string_file for the command output.
	* ui-file.c (term_cli_styling): New function (most code moved
	from utils.c can_emit_style_escape).
	(string_file::string_file, string_file::can_emit_style_escape,
	stdio_file::can_emit_style_escape, tee_file::term_out,
	tee_file::can_emit_style_escape): New functions.

gdb/stack.c

@@ -2693,7 +2693,8 @@ frame_apply_command_count (const char *which_command,
 	       set to the selected frame.  */
 	    scoped_restore_current_thread restore_fi_current_frame;
 
-	    cmd_result = execute_command_to_string (cmd, from_tty);
+	    cmd_result = execute_command_to_string
+	      (cmd, from_tty, gdb_stdout->term_out ());
 	  }
 	  fi = get_selected_frame (_("frame apply "
 				     "unable to get selected frame."));