MfP4: Handle entries with unset size properly: Regular files with

MfP4: Handle entries with unset size properly: Regular files with
unspecified size are "unlimited" (required by Zip reader, which
sometimes does not know the uncompressed size of an entry until it
gets to the end).  Also, hardlinks with unspecified (or zero) size do
not overwrite the data on disk nor do they set metadata.  This is
compatible with GNU tar and NetBSD pax behavior.