Add a stage-qa check to check for the existence of SONAME in .so's.

If a port provides .so.X files, they have to have a SONAME for them to
work correctly.

While there, incorporate a reverse soname checks in proxydeps.
A port that needs a .so.X from another port which does not have a
SONAME.

Reviewed by: bapt
Sponsored by: Absolight
Differential Revision: https://reviews.freebsd.org/D7454