doc/license updates

Reword the vimpagerrc section in vimpager.md to reflect the different
possible locations including ~/.vim/vimpagerrc introduced in 2ae0a3f .

Add the 3-clause BSD license to vimcat and vimcat.md, I would have liked
to use the 2-clause license like in vimpager but the original vim script
upon which vimcat is based uses the 3-clause license.

Add my email address to the copyright lines.

Change Makefile to not use install -D, because OSX does not have install
-D, use mkdir -p instead before the install invocations.