env: Move env_set() out of cmd/nvedit.c and in to env/common.c

env: Move env_set() out of cmd/nvedit.c and in to env/common.c

Inside of env/common.c we already have our helper env_set_xxx functions,
and even have a comment that explains why env_set() itself wasn't moved.
We now handle that move. This requires that we rename the previous
_do_env_set() to env_do_env_set() and note it as an internal env
function. Add comments about this function to explain why we do this
when we add the prototype. Add a new function, env_inc_id() to allow for
the counter to be updated by both commands and callers, and document
this as well by the prototype.

Signed-off-by: Tom Rini <trini@konsulko.com>