Fix hostrange_hn_within to allow for mixed prefix lengths.

While here, mark options const, and add leading underscore to denote this
as a static function (only called within hostlist.c). Also change strcmp
to xstrcmp.

Commit a6ffef22 changed this function and would alter the input hn,
which led to subsequent calls to the function having wrong prefix
lengths for that hostrange precluding it from matching correctly.

Bug 2558.