overhaul mbsrtowcs

these changes fix at least two bugs:
- misaligned access to the input as uint32_t for vectorized ASCII test
- incorrect src pointer after stopping on EILSEQ

in addition, the text of the standard makes it unclear whether the
mbstate_t object is to be modified when the destination pointer is
null; previously it was cleared either way; now, it's only cleared
when the destination is non-null. this change may need revisiting, but
it should not affect most applications, since calling mbsrtowcs with
non-zero state can only happen when the head of the string was already
processed with mbrtowc.

finally, these changes shave about 20% size off the function and seem
to improve performance by 1-5%.
fix incorrect range checks in wcsrtombs

negative values of wchar_t need to be treated in the non-ASCII case so
that they can properly generate EILSEQ rather than getting truncated
to 8bit values and stored in the output.
minor optimization to mbstowcs

there is no need to zero-fill an mbstate_t object in the caller;
mbsrtowcs will automatically treat a null pointer as the initial
state.
cleanup mbstowcs wrapper

remove unneeded headers. this file is utterly trivial now and there's
no sense in having a comment to state that it's in the public domain.
cleanup wcstombs

remove redundant headers and comments; this file is completely trivial
now. also, avoid temp var.
add put*ent functions for passwd/group files and similar for shadow

since shadow does not yet support enumeration (getspent), the
corresponding FILE-based get and put versions are also subbed out for
now. this is partly out of laziness and partly because it's not clear
how they should work in the presence of TCB shadow files. the stubs
should make it possible to compile some software that expects them to
exist, but such software still may not work properly.
eliminate gcc dependency for testing char signedness in limits.h
eliminate bits/wchar.h

the preprocessor can reliably determine the signedness of wchar_t.
L'\0' is used for 0 in the expressions so that, if the underlying type
of wchar_t is long rather than int, the promoted type of the
expression will match the type of wchar_t.
fix type issues in stdint.h so underlying types of 64-bit types match ABI
add arpa/tftp.h