Avoid use of stat() in procheader_parse_file() on Windows.

Avoid use of stat() in procheader_parse_file() on Windows.

Since GLib on different Windows versions has different
ideas about sizes of struct stat field sizes and is
inconsistent about it on 64-bit systems, we take advice
from GLib's own gstdio.h, and use GIO API to get file
attributes instead.

This will likely have to be changed for all uses of
stat() or g_stat() elsewhere.

All this to stop Claws Mail showing "1GB" size for all
emails in summaryview on 64-bit Win10.