Allow several extra library and plugin directories (libdir option)
Make getlibdirs return an array (or whathever this type is called in perl)
Merge remote-tracking branch 'spalax/paternal/libdirs'
Simplify libdirs: libdirs must be plural, libdir must be a single string

This makes the documentation read more sensibly, and matches how we
handle underlaydirs and underlaydir.
changelog, close bug
Add regression test for libdir/libdirs