aft: Store resolved paths when adding files.

Currently the add command performs some sanity checks on the given
paths, but stores them unmodified in the audio file table if they
pass the checks. This commit removes the checks in favor of a call to
realpath(3) to get the canonicalized absolute pathname to be stored
in the audio file table.

The new code relies on POSIX.1-2008 semantics. That is, it calls
realpath() with second argument NULL to let the function allocate a
suitably sized buffer. This should not be a problem, since the old
POSIX.1-2001 version is broken by design, and all moderately recent
systems support the new semantics.

This change breaks t0004 which expects the ls -p command to print
the same paths that were used earlier to add audio files, which is
no longer true. We fix the test by simply running ls without the
-p option.