revoke(2): Use unix socket externalize code to handle revoke.

revoke(2): Use unix socket externalize code to handle revoke.

Use revoke token in shared mode in unix socket code and in exclusive
mode in fdrevoke(); mainly to make sure that all fps externalized and
marked FREVOKED will be picked up by later allproc_scan() called by
fdrevoke().

This one greatly simplies the code on unix socket side.  The original
unix socket revoke handling was also kinda broken: it tried to hold
socket reception buffer token w/ all filedesc spin lock being held.

Go-Ahead-by: dillon@