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.

fix return value computation in one code path of wcsnrtombs

the affected code was wrongly counting characters instead of bytes.

include cleanups: remove unused headers and add feature test macros

use restrict everywhere it's required by c99 and/or posix 2008

to deal with the fact that the public headers may be used with pre-c99
compilers, __restrict is used in place of restrict, and defined
appropriately for any supported compiler. we also avoid the form
[restrict] since older versions of gcc rejected it due to a bug in the
original c99 standard, and instead use the form *restrict.

fix longstanding exit logic bugs in mbsnrtowcs and wcsnrtombs

these are POSIX 2008 (previously GNU extension) functions that are
rarely used. apparently they had never been tested before, since the
end-of-string logic was completely missing. mbsnrtowcs is used by
modern versions of bash for its glob implementation, and and this bug
was causing tab completion to hang in an infinite loop.

initial check-in, version 0.5.0