gdb: use std::string for internalvar::name

Change internalvar::name to std::string, automating memory management.
It becomes necessary to allocate internalvar with new instead of XNEW.

I didn't find how to trigger the code in complete_internalvar.  It is
called from condition_completer, so it should be by using the
"condition" command, but I never managed to get in the right code path.

Change-Id: I814d61361663e7becb8f3fb5f58c0180cdc414bc
Approved-By: Tom Tromey <tom@tromey.com>