Log Message:
net/unison*: Rename unison2.51 to unison

During the 2.51 branch, upstream decided that unison will have
backwards compatibility, so that there is no longer any reason to run
old versions.  With 2.52, not only can it interop with 2.51 (and
2.48!), but it does not need to have matching ocaml version when
interoperating with other 2.52.x or newer.  Thus, unison will now have
a single version in pkgsrc, and this is expected to continue.