Previously we were protecting access to zheadSockTab with zheadMu
because this table was accessed from only two places: when opening
.wcfs/zhead and in zwatcher. Soon we are going to add another place that
will access this table and still using big zheadMu seem less and less
logical.

-> Switch to using dedicated lock to protect table of .wcfs/zhead opens
   as preparatory step for that.