bootspec: move augmentation of loader-discovered entries into bootspec.c

bootspec: move augmentation of loader-discovered entries into bootspec.c

Previously, bootctl would show boot loader entries discovered by the
boot loader which couldn't found locally separately in the output.

Let's move this code into bootspec.c, and beef it up a bit. This way we
can use it later on for logind, and correctly show automatically
discovered windows/macos entries too.