consolidate str[n]casecmp_l into str[n]casecmp source files

this is mainly done for consistency with the ctype functions and to
declutter the src/locale directory.

fix all implicit conversion between signed/unsigned pointers

sadly the C language does not specify any such implicit conversion, so
this is not a matter of just fixing warnings (as gcc treats it) but
actual errors. i would like to revisit a number of these changes and
possibly revise the types used to reduce the number of casts required.

initial check-in, version 0.5.0