Improve encoding detection of wimupdate command files

Make wimupdate command files (read from stdin) and wimcapture source
list files be interpreted as UTF-8 or UTF-16LE (autodetected).
Previously these files were interpreted as the Windows codepage or
UTF-16LE on Windows (autodetected), and UTF-8 on non-Windows.

This makes these files be consistent with wimextract pathlist files and
wimcapture config files, which already had the behavior of "autodetect
UTF-8 or UTF-16LE on all platforms".

Resolves https://wimlib.net/forums/viewtopic.php?p=1545