debian stretch: if /usr/bin/gpg is a symlink, bitmask fails to initiate

default can be made using dpkg-divert: http://superuser.com/questions/763724/how-to-set-gpg2-as-default-implementation-of-gpg-on-debian

right now, we don't pick any path that is a symlink. if /usr/bin/gpg is a symlink, we still can try /usr/bin/gpg1 or /usr/bin/gpg2

(from redmine: created on 2015-11-02, closed on 2016-02-18, precedes #7564)