Fix named when using plugins and chroot.

BIND9 introduced plugins and migrated the filter-aaaa feature to a
plugin.
As it loads its plugins late in the startup process (read after chroot),
the plugins need to be available in the chroot.

Also, refactor the code now that a second directory need to be handled.

PR: 238011
Reported by: ryan@timewasted.me
MFH: 2019Q2