Change gdb_realpath to return a unique_xmalloc_ptr
This changes gdb_realpath to return a unique_xmalloc_ptr and fixes up
the callers. This allows removing some cleanups. This change by
itself caused xfullpath.exp to fail; and attempting to fix that ran
into various problems (like .get() being optimized out); so this patch
also rewrites xfullpath.exp to be a C++ selftest instead.