Use std::string and unique_xmalloc_ptr in demangle_command

Use std::string and unique_xmalloc_ptr in demangle_command

Change demangle_command to use std::string and unique_xmalloc_ptr,
removing some cleanups.

ChangeLog
2017-09-03  Tom Tromey  <tom@tromey.com>

	* demangle.c (demangle_command): Use std::string,
	unique_xmalloc_ptr.