add `forbidden_linkall` as an optional field in executable(s) stanza

In the same style as forbidden_libraries, it'd be useful to have forbidden_linkall to guarantee an executable is not compiled with linkall.

It turns out dune can add the linkall option if some libraries (notably dune-site, but I don't know if there are others) are in the dependency graph (even transitively). Because of this, the linkall flag can creep up into an executable when a new dependency is added or when a dependency is updated (e.g., camomile<2 and camomile≥2).

Expected behaviour

With a stanza

(executable
 (forbidden_linkall)
 …)

is evaluated. The build fails if the executable would be compiled with linkall.