Corrected exit for `check' target in Makefile.in
FAILLOC now determines type using typeof; removed first argument and FAILLOC_PTR
Added tests for wmbc_str{,n}cpy, along with function improvments/fixes

In particular, both functions will now return const char * to discourage using
the returned pointer to modify the buffer. Additionally, NULL will now be
returned in the event of an expansion failure, otherwise the returned pointer
would be one byte outside of the buffer.