Install named plugins into a separate directory

named plugins are currently installed into ${libdir}, which is convenient because that is where dlopen() looks by default when supplied a filename without path separators. However, this practice clutters ${libdir} and makes it hard for packagers to determine what goes into which package.

AFAIK, it is customary for software packages to install their plugins into a separate directory inside ${libdir}, so that is arguably what we should do, too.

There are platform-specific methods of making dlopen() look where we tell it to for filenames without path separators, but troubleshooting any problems with these methods would likely be a right pain in the neck.

I think we should address this before 9.14 is stabilized due to filter-aaaa.so already being built in development releases (it is installed into /usr/lib or /usr/lib64 and thus, for some extra confusion, on RHEL/CentOS it may land in the *-libs package instead of the main one).

Installing plugins into a separate directory will also make creating a potential future bind-plugins package a breeze.