Add relXXY macros to be used by other macros, i.e., relX.current.

This is mentioned in a comment in the file, but more work on this
file is planned, likely as part of the 11.3 announcement commit.

The idea behind this is rel.current and rel1.current (and others)
will include rel120.current (for 12.0) and rel112.current (for 11.2)
in order to reduce the number of files touched in the tree when
a new release is available, or a release reaches end-of-life.

Sponsored by: Rubicon Communications, LLC (Netgate)