Add deferred creation for dangerous symlinks.

Symbolic links with targets that are absolute paths or contain ".."
are potentially dangerous, as they could be used to point to
arbitrary directories within the filesystem. Add protection against
this by deferring their creation to the end of extraction. This is
the same scheme used by GNU tar.
Fix some corner cases with symlink handling.
Add better symbolic link tests.

The non-free Unix LHA tool has some weird corner cases in how it
stores symbolic links containing targets with path separators. Add
test archives for this.
Only show symlink extraction message once.

The new deferred symlink mechanism means that dangerous symlinks are
returned twice by lha_reader_next_file. Add an API so that the "fake"
ones can be identified, and silence the second message in the command
line tool.