Fix Windows build warnings (#131487)

Fix the following warnings:

* Modules\_io\fileio.c(1296,13): unused variable 'self'
* Modules\_io\winconsoleio.c(334,9): unused variable 'fd_is_own'
* Modules\faulthandler.c(409,11): unused variable 'flags'
* Modules\posixmodule.c(5699,9): unused variable 'pathError'
* PC\winreg.c(2077,5): suggest braces around initialization of
  subobject
* PC\winreg.c(34,13): unused variable 'errNotAHandle'
* Python\fileutils.c(132,12): result of comparison of constant
  1114111 with expression of type 'wchar_t' (aka 'unsigned short') is
  always false
* Python\fileutils.c(58,21): unused variable 'INCOMPLETE_CHARACTER'
* Python\sysmodule.c(2534,21): unused variable 'perf_map_state'