Add debug output about skipping files and functions

Add debug output about skipping files and functions

While trying to create skips for libstdc++, I found myself debugging GDB
quite a bit, mostly to find out what the exact function name to match
is.  I thought it would make sense to have this information as debug
output.

This patch adds "set debug skip on|off".

gdb/ChangeLog:

	* skip.c (debug_skip): New variable.
	(skiplist_entry::do_skip_file_p): Add debug output.
	(skiplist_entry::do_skip_gfile_p): Likewise.
	(skiplist_entry::skip_function_p): Likewise.
	(_initialize_step_skip): Create debug command.
	* NEWS: Mention set/show debug skip.

gdb/doc/ChangeLog:

	* gdb.texinfo (Skipping Over Functions and Files): Document
	set/show debug skip.