Remove cleanups from tracefile.c

Remove cleanups from tracefile.c

This removes cleanups from tracefile.c, by introducing a unique_ptr
specialization.

This code could be made even simpler via a deeper C++-ification, but I
have not attempted that.

Tested by the buildbot.

gdb/ChangeLog
2018-06-10  Tom Tromey  <tom@tromey.com>

	* tracefile.c (struct trace_file_writer_deleter): New.
	<operator()>: Rename from trace_file_writer_xfree.
	(trace_file_writer_up): New typedef.
	(tsave_command, trace_save_tfile, trace_save_ctf): Update.