revise profile machinery

[mobileink]

The fact that profiles are implemented as toolchains is an implementation detail, so remove "toolchain" from the relevant names.

E.g. rename @opam.ocamlsdk//toolchain/profiles to something like @opam.ocamlsdk//profile/selectors so we get

register_toolchains(
    "@opam.ocamlsdk//toolchain/selectors/local:all",
    "@opam.ocamlsdk//profile/selectors:all",

instead of the current

register_toolchains(
    "@opam.ocamlsdk//toolchain/selectors/local:all",
    "@opam.ocamlsdk//toolchain/profiles:all",

Ditto for the profile rules.