Permit csh/fish to be overridden via an explicit `opam config env --sh`.

Followup to a11b712225c1a56f5bf3edbb2aaf7f865ec53a11
fix grammar in various man entries, and use 'OPAM' instead of 'opam' consistently to refer to it.
I think POAM is more catchy than OPAM, but @samoht disagrees!
Merge pull request #722 from avsm/master

Permit csh/fish to be overridden via an explicit `opam config env --sh`.