Rewriting description of option -no-aspcud, so that it does not mention aspcud.
Still keeping the name -no-aspcud, though, for backwards compatibility
Added alias --use-internal-solver to --no-aspcud, which is deprecated.
Added environment variable OPAMUSEINTERNALSOLVER corresponding to the option --use-internal-solver
Merge pull request #1277 from rdicosmo/add-solver-option

Add --use-internal-solver option, keep --no-aspcud as an alias