[dev & wp] fix install

See merge request frama-c/frama-c!2388

 - deprecation messages for alt[gr]-ergo and coq[ide]
 - require why3 >= 1.2.0 (includes conflicts with why3-base)
 - no more need for why3-coq

Update opam stage instruction in release manual

See merge request frama-c/frama-c!2399

[libC] fixes specification of abort and exit functions

Closes #684

See merge request frama-c/frama-c!2331

Handles \exit_status exactly as \result.

Fixes indentation for ocp-indent 1.7

See merge request frama-c/frama-c!2369

[wp] use ./.frama-c/wp for cache & scripts

See merge request frama-c/frama-c!2395

Remove unused options in WP parameters

See merge request frama-c/frama-c!2390