Compression agnostic symlink for pkg

[zac_freebsdfoundation.org]

Summary

Create a compression format agnostic symlink under the name pkg.bootstrap, as suggested by @kevans in D26517.

Test Plan

Create directory /tmp/pkgs, run make -DBATCH PACKAGES=/tmp/pkgs -C ports-mgmt/pkg package in the root of the repository. The output shows both the old and new symlink being created:

install -l rs /tmp/pkgs/All/pkg-1.15.9.txz /tmp/pkgs/Latest/pkg.txz
install -l rs /tmp/pkgs/All/pkg-1.15.9.txz /tmp/pkgs/pkg.bootstrap