a new function dup_shell() to replace ok_shell() and make it unconditionnally
strdup() its result to make the caller's code simplier. Change ok_shell() to
just return an integer value suitable for tests (it was used mainly for that
purpose). Do not use strdup() in the caller's code but rely on dup_shell()
that will do the job for us.
PR: bin/2442
296d834419