Do not return a handle to a port when searching by name

The standard explicitly states that only object with a full name
can be searched for by name. A port does not have a full name and
hence should be skipped so that a different object (the signal,
etc.) can be returned. This patch adds code to skip ports when
searching for an object handle by name.
Merge branch 'master' of git://github.com/steveicarus/iverilog