in parse_syntax(), don't generate an error if we find a duplicate syntax

in parse_syntax(), don't generate an error if we find a duplicate syntax
name, since we might be trying to override a syntax in the global nanorc
with one in our local nanorc; instead, free any duplicate syntaxes we
find, so that we always use the last syntax with a given name

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@3382 35c25a1d-7b9e-4130-9fde-d3aeb78583b8