Allow last account mask to be deleted with odelmask.

src/nickserv.c (nickserv_delmask): Add parameter to distinguish between
    normal and override use.
  (cmd_delmask): Pass the new argument.
  (cmd_odelmask): Likewise.
git-archimport-id: srvx@srvx.net--2006/srvx--devo--1.3--patch-78