biology/ucsc-userapps: Add "kent" command in ${PREFIX}/bin

Standard kent userapps commands are installed under ${PREFIX}/userapps
because of multiple conflicts. The kent command turns the standard
commands into subcommands, so no special PATH is needed.

/usr/local/ucsc-userapps/bin/bigWigToBedGraph [args]

can be run as

kent bigWigToBedGraph [args]