casa-mp-base

haskell-stack: Rename to haskell_stack, extend

Extend the haskell-stack 1.0 PortGroup with livecheck functionality,
switch to setting defaults instead of setting the options directly, use
port::register_callback to ensure the added dependencies are not
accidentally overwritten, provide haskell_stack.system_ghc to switch to
system GHC instead of downloading a copy.

Adjust the git-annex, shellcheck, pandoc and adblock2privoxy Portfiles
and drop functionality from the Portfiles that is now available in the
PortGroup.