clamp read and write length to INT_MAX in win32 posix shims

[sage-mode-hunter]

posix_read/posix_write are the process-wide read()/write() on Windows (win32netcompat.h maps read/write to them) and pass their size_t count straight into the int-length socket and CRT calls:

• recv/send take an int length, so a count at or past 2 GiB is narrowed at the call
• count == 4 GiB truncates to 0, so recv() returns 0 and the read loop reads it as a clean EOF
• 2 GiB truncates to INT_MIN and fails a valid buffer with WSAEFAULT; 4 GiB + 1024 sends 1024 and reports it as complete
  Clamp count to INT_MAX in both shims before the call. POSIX allows read/write to move fewer bytes than requested so callers loop for the rest, and normal-sized I/O is unchanged.