Add basic infrastructure for storing xattr items

Add basic infrastructure for storing xattr items

Define a new tagged metadata item to hold a list of names and values of
Linux-style extended attributes, and prepare for supporting
capture/apply of extended attributes.

I considered making the xattrs a stream instead, referenced from the
tagged item which would just hold a hash.  This would have allowed
xattrs to be deduplicated between files.  However, I ultimately decided
against this because WIMGAPI and older versions of wimlib would discard
the streams on optimize/export, and extraction would be much more
complicated because xattr streams could come up for extraction before
other streams --- which would be especially problematic for symlinks.