We now allow multiple paths (files/directories) to be archived in
one shot. Validation of the target path(s) is now delegated to the
actual archive helper.
* archive: fix unrar check when using unrar-free
unrar-free returns the error code 1 when run without arguments, thus
failing the presence check. Replacing the current presence check with
(( $+commands[unrar] )) fixes the problem.
* archive: add unar support for lsarchive and unarchive
No `archive` support with unar.
unrar and rar will discard everything up to the file name when extracting
with `e`. This breaks extraction of archives that have files with duplicate
file names and ones where the paths are important. The `x` command
extracts with the full path instead.
