devel/arcanist: Split into arcanist-lib and arcanist.

The arcanist port has always conflicted with archivers/arc, but
if the port is installed without the symlink in PREFIX/bin, there
is no issue.

A user expecting to be able to use arcanist in this way can alias
arcanist in their .cshrc or similar and only install the -lib port.

Approved by: grembo (maintainer)
Differential Revision: https://reviews.freebsd.org/D19920