replace all instances of strncpy() with charcpy(), since the only

replace all instances of strncpy() with charcpy(), since the only
difference between them is that the former pads strings with nulls when
they're longer than the number of characters specified, which doesn't
appear to be used anywhere

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@2542 35c25a1d-7b9e-4130-9fde-d3aeb78583b8