Add ABI compatability aliases.

GNU used several extensions that were incompatible with C99 and POSIX,
so they used alternate names for the standard functions.

The result is that we need these to run standards-conformant programs
that were linked with glibc.

clean up stdio_impl.h

this header evolved to facilitate the extremely lazy practice of
omitting explicit includes of the necessary headers in individual
stdio source files; not only was this sloppy, but it also increased
build time.

now, stdio_impl.h is only including the headers it needs for its own
use; any further headers needed by source files are included directly
where needed.

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 failure to distinguish input/match failure in wide %[ scanf

this also includes a related fix for vswscanf's read function, which
was returning a spurious (uninitialized) character for empty strings.

introduce new wide scanf code and remove the last remnants of old scanf

at this point, strto* and all scanf family functions are using the new
unified integer and floating point parser/converter code.

the wide scanf is largely a wrapper for ordinary byte-based scanf;
since numbers can only contain ascii characters, only strings need to
be handled specially.

initial check-in, version 0.5.0