Without this patch, the following error is emitted when compiling with the latest LLVM/Clang 8.x:
```text
src/stdio/vfwscanf.c:218:9: error: assigning to 'const wchar_t *' (aka 'const unsigned int *') from 'int [1]' converts between pointers to integer types with different sign [-Werror,-Wpointer-sign]
set = L"";
^ ~~~
1 error generated.
```
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.
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.
shunget cannot unget eof status, causing wcstol to leave endptr
pointing to the wrong place when scanning, for example, L"0x". cheap
fix is to make the read function provide an infinite stream of bogus
characters rather than eof. really this is something of a design flaw
in how the shgetc system is used for strto* and wcsto*; in the long
term, I believe multi-character unget should be scrapped and replaced
with a function that can subtract from the f->shcnt counter.
