Adds an utility function to make it shorter to write the common case
of wrapping an xstrdup with a unique_xmalloc_ptr, and uses it
throughout.
Note: I tried to put this in common/common-utils.h near skip_spaces,
etc. but that is included in common/common-defs.h before
common/gdb_unique_ptr.h is included, so it would fail to compile
because gdb::unique_xmalloc_ptr isn't defined at that point yet. I
tried moving the gdb_unique_ptr.h inclusion before common-utils.h, but
that doesn't work because gdb_unique_ptr.h depends on common-utils.h
for xfree.
gdb/ChangeLog:
2019-06-04 Pedro Alves <palves@redhat.com>