[Toybox] Thoughts on seperating shell dependencies and MAYFORK commands?

[Oliver Webb]

When doing "make sh", scripts/single.sh looks for MAYFORK commands to pull in as builtin's
Which means any command that is declared with MAYFORK is automatically included into the shell when doing "make sh".
TOYFLAG_MAYFORK is essentially "if we are calling this in the shell, don't fork/exec to save system resources"

There is a pretty large distinction between "I'd like this to be automatically put in the shell when doing 'make sh'"
and "I'd like to have this be used by the shell instead of forking if it's in the same toybox binary as it"

Commands like cat, ls, mv, cp, du, find, rm, etc would benefit by being MAYFORK commmands,
But it also would not make sense to automatically include them into a single command binary of the shell.

The solution to this, that would give a multicommand binary with a shell the ability to run faster by not forking off
processes. And a single command binary of the shell the ability to have commands like ':' without pulling in things like
find or rm, would be to create 2 flags with the same value, and only scan for one in scripts/single.sh.
I.e. changing the existing MAYFORK declarations to something else (TOYFLAG_SHELLDEP, maybe TOYFLAG_BUILTIN?).
Scanning for _that_ instead of MAYFORK in scripts/single.sh, and adding a declaration of it in lib/toyflag.h.

Thoughts? I already have this working, there isn't any build infrastructure I know of that breaks when you do this, 
and the only reason I am not sending a patch yet is because I dunno a actual good name for the flag

-   Oliver Webb <aquahobbyist at proton.me>