Core: fixed stderr redirection on win32 in ngx_reopen_files().

Core: fixed stderr redirection on win32 in ngx_reopen_files().

On win32 stderr was not redirected into a file specified by "error_log"
while reopening files.  Fix is to use platform-independent functions to
work with stderr, as already used by ngx_init_cycle() and main() since
rev. d8316f307b6a.