generator: add always-available optgroups

Support the possibility to have optional groups always enabled (e.g.
because they were present in the past, and they need to be kept for
users).
Add and use few helper optgroups-related functions to deal also with
them.
Make realpath mandatory

commit a86eb0e0d2c67e2153139c681632edc72162b8ed made it an optional
feature, as on Windows it was not available; on the other hand, realpath
has been used unconditionally already for quite some time, so having
just the "realpath" command conditional on the presence of it does not
make much sense.

Drop the configure/build system handling of it, make the "realpath"
command no more optional, and keep the "realpath" feature as
unconditionally available now.
daemon: add sysroot_realpath

Similar to sysroot_path, but first resolves (using realpath) the given
path within sysroot.
daemon: scrub-file: resolve the path before calling scrub (RHBZ#1099490).

Resolve the given path within the chroot, so scrub can be invoked
outside the chroot on an already-resolved path.

Add few tests in scrub-file for this and other similar issues.