remove comments on copyright status from UTF-8 implementation files

despite clarifications made to the COPYRIGHT file in commit
f0a61399330bae42beeb27d6ecd05570b3382a60, there continues to be
confusion about whether the permissions granted actually apply to all
files. I am the sole author of these files and clearly intend, and
have always intended, for the grant of permission to apply to them.

include cleanups: remove unused headers and add feature test macros

fix failure of mbsinit(0) (not UB; required to return nonzero)

issue reported by Richard Pennington; slightly simpler fix applied

initial check-in, version 0.5.0