Introduce the new BINARY_ALIAS variable

When defined it will create symlinks of some given binaries in a directory which
will be prepended to the PATH.

The syntax is the following:
BINARY_ALIAS= target1=source1 target2=source2

For example to have a "swig" binary in the path which will be pointing at
swig3.0 and a "sed" pointing at GNU sed: gsed

BINARY_ALIAS= swig=swig3.0 sed=gsed

Reviewed by: swills, adamw, mat
Approved by: swills (portmgr)
Differential Revision: https://reviews.freebsd.org/D12603