p2phttp: Scan multilevel directories with --directory

p2phttp: Scan multilevel directories with --directory

This allows for eg dir/user/repo structure. But also other layouts. It
still does not look for repositories that are nested inside other
repositories.

The check for symlinks is mostly to avoid cycles that would prevent
findRepos from returning. Eg, foo/bar/baz being a symlink to foo/bar.

If the directory is writable by someone else they can still race it and
get it to follow a symlink to some other directory. I don't think p2phttp
needs to worry about that kind of situation though, and I doubt it avoids
such problems when operating on files in a git-annex repository either.