Do not emit style escape sequences to log file

Do not emit style escape sequences to log file

PR gdb/24502 requests that the "set logging" log file not contain
style escape sequences emitted by gdb.

This seemed like a reasonable request to me, so this patch implements
filtering for the log file.

This also updates a comment in ui-style.h that I noticed while writing
the patch.

Tested on x86-64 Fedora 29.

gdb/ChangeLog
2019-06-14  Tom Tromey  <tromey@adacore.com>

	PR gdb/24502:
	* ui-style.h (skip_ansi_escape): Update comment.
	* ui-file.h (class no_terminal_escape_file): New class.
	* ui-file.c (no_terminal_escape_file::write)
	(no_terminal_escape_file::puts): New methods.
	* cli/cli-logging.c (handle_redirections): Use
	no_terminal_escape_file.

gdb/testsuite/ChangeLog
2019-06-14  Tom Tromey  <tromey@adacore.com>

	PR gdb/24502:
	* gdb.base/style-logging.exp: New file.