unit-file: tighten unit file discovery checks

Only accept DT_REG/DT_LNK/DT_DIR entries, ignore all others.

Only accpet DT_REG/DT_LNK for file names that are valid unit file names.

Only accept DT_DIR for filenames that are valid unit file names which
are suffixed by .d, .wants, .requires

This doesn't really fix any bugs, but tightens what we insert into the
lookup tables.