libalias: tidy up housekeeping

Replace current expensive, but sparsly called housekeeping
by a single, repetive action.

This is part of a larger restructure of libalias in order to switch to
more efficient data structures. The whole restructure process is
split into 15 reviews to ease reviewing. All those steps will be
squashed into a single commit for MFC in order to hide the
intermediate states from production systems.

Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D30277

(cherry picked from commit 294799c6b081faece556a5010a4f51552e131c2f)