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Scan the buffer of a FILE in bulk with glibc and the Apple C library (#6030)

* Scan the buffer of a FILE in bulk with glibc and the Apple C library

Parsing from a FILE* still read one character at a time through
getc_unlocked(), while a std::istream has its get area scanned in bulk.
file_input_adapter now offers the lexer the same kind of window into the
buffer of the FILE: it reads the fields that the C library's own
getc_unlocked() macro reads (glibc: _IO_read_ptr and _IO_read_end; Apple:
_p and _r) and consumes exactly the characters getc_unlocked() would
return, so the file is left where it was left before. The part of the
current token in a buffer that is about to be refilled is kept for error
messages, as for streams. The window needs the file locked for the whole
parse, so JSON_DISABLE_UNLOCKED_FILE_IO and MemorySanitizer builds turn it
off; uClibc, which defines __GLIBC__ but has other fields, does not get it.

parse() from a FILE* gets 2.0-2.6x as fast on an Apple M1 Max and 1.7-2.2x
on x86-64 (glibc, GCC 15 and Clang 21), accept() 2.9-4.4x; that is about as
fast as parsing from a std::ifstream. The new tests read with FILE buffers
of 1 to 70 bytes and unbuffered files, after ungetc(), with SAX positions
and with binary strings across the buffer's edges.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Tests: no setvbuf() buffer below 2 bytes on Windows

The Microsoft C runtime rejects a _IOFBF buffer size below 2 as an
invalid parameter and ends the process (0xc0000409), which made
test-file_input fail on all MSVC and clang-cl jobs.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Fix CI: astyle formatting in unit-file_input.cpp

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Tests: limit the multi-character ungetc() test to glibc and Apple

The Windows C runtime only supports one pushed-back character, which
broke the Windows CI jobs.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/co... (continued)

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