[Toybox] CONFIG_TOYBOX_ZHELP

[enh]

this will hardly come as a surprise to you, but just fyi, i _haven't_
enabled this for AOSP, (a) to avoid building deflate.c (which would
make it harder to recognize if we accidentally stopped using "real"
zlib[1]), and (b) because we check in the generated files, and i'm not
likely to check in a binary blob, even if it is encoded in ASCII as a
c byte array :-)

thanks for keeping the uncompressed path!

i haven't built with it enabled on linux yet, but building for macOS
with it enabled at least i saw this spam for every (?) toy:
```
In file included from toys/posix/who.c:26:
In file included from ./toys.h:8:
./generated/config.h:695:9: warning: 'CFG_TOYBOX_ZHELP' macro
redefined [-Wmacro-redefined]
#define CFG_TOYBOX_ZHELP 0
        ^
./generated/config.h:691:9: note: previous definition is here
#define CFG_TOYBOX_ZHELP 1
        ^
./generated/config.h:696:9: warning: 'USE_TOYBOX_ZHELP' macro
redefined [-Wmacro-redefined]
#define USE_TOYBOX_ZHELP(...)
        ^
./generated/config.h:692:9: note: previous definition is here
#define USE_TOYBOX_ZHELP(...) __VA_ARGS__
        ^
```

____
1. yes, i'm acutely aware there are far too many zlib forks, but ykwim :-)