unsupported calls: use undef function exception

Returning a "plain" error tuple for unsupported calls breaks the type
spec for functions:

    On BSD:
        -spec setproctitle(alcove_drv:ref(),iodata()) -> 'ok'.

    On Linux and Solaris:
        -spec setproctitle(alcove_drv:ref(),iodata()) -> {'error','unsupported'}.

So the type spec would need to be amended to be the union of both return
values and portable code would have to test for both cases, with the
effect that any call in the future might return 'unsupported' if alcove
were ported to a new OS.

Attempting to make an supported call will now result in an "undefined
function" exception:

    1> catch alcove:setproctitle(P, "foo").
    {'EXIT',{undef,[{alcove,call,
                            [<0.46.0>,setproctitle,["foo"]],
                            [{file,"src/alcove.erl"},{line,426}]},
                    {erl_eval,do_apply,6,[{file,"erl_eval.erl"},{line,661}]},
                    {erl_eval,expr,5,[{file,"erl_eval.erl"},{line,434}]},
                    {shell,exprs,7,[{file,"shell.erl"},{line,684}]},
                    {shell,eval_exprs,7,[{file,"shell.erl"},{line,639}]},
                    {shell,eval_loop,3,[{file,"shell.erl"},{line,624}]}]}}
    2> os:type().
    {unix,linux}

This still places a burden on the caller. Portable code either needs to
hardcode supported functions by OS:

    % And imagine all the error checking this code is not doing ...
    proctitle(Name) ->
        case os:type() of
            {unix,linux} -> alcove:prctl(...);
            {unix,BSD} where BSD = ... -> alcove:proctitle(...)

Or run through sequences of awkward try/catch statements:

    proctitle(Name) ->
        try alcove:proctitle(...) of
            ok -> ok
        catch
            error:undef ->
                try alcove:prctl(...) of
                    {ok,_,_,_,_,_} -> ok
                catch
                    error:undef ->
                        % Does Solaris even?
                        ...
                end,
        end

Dealing with portability should be the job of a higher level library
built on top of alcove.