C++-ify skip.c

C++-ify skip.c

I happened to notice that skiplist_entry, in skip.c, contains a
gdb::optional<compiled_regex> -- but that this object's destructor is
never run.  This can result in a memory leak.

This patch fixes the bug by applying a bit more C++: changing this
code to use new and delete, and std::unique_ptr; and removing cleanups
in the process.

Built and regression tested on x86-64 Fedora 25.

ChangeLog
2017-08-09  Tom Tromey  <tom@tromey.com>

	* skip.c (skiplist_entry): New constructor.
	(skiplist_entry::enabled, skiplist_entry::function_is_regexp)
	(skiplist_entry::file_is_glob): Now bool.
	(skiplist_entry::file, skiplist_entry::function): Now
	std::string.
	(make_skip_entry): Return a unique_ptr.  Use new.
	(free_skiplist_entry, free_skiplist_entry_cleanup)
	(make_free_skiplist_entry_cleanup): Remove.
	(skip_command, skip_disable_command, add_skiplist_entry)
	(skip_form_bytes, compile_skip_regexp, skip_command, skip_info)
	(skip_file_p, skip_gfile_p, skip_function_p, skip_rfunction_p)
	(function_name_is_marked_for_skip): Update.
	(skip_delete_command): Update.  Use delete.