Documenting CONFIG_EXTRA_COMPAT better

[Rich Felker]

Hi,

I'm wondering if it would be acceptable to change the name and
description of CONFIG_EXTRA_COMPAT a bit. It regularly comes up among
musl users who find that busybox does not build against musl with this
option enabled, and who wonder whether it's something important they
would be missing if they turn it off.

The basic proposed changes I have in mind would be:

- Naming it something like "support matching embedded NUL bytes with
  regular expressions". (Or is there anything else useful it does?)

- Including in the description that it changes several tools to use
  the GNU regex API instead of the POSIX regex API, and maybe
  mentioning explicitly that the GNU API is available with glibc and
  uClibc but not musl. (Well, maybe not even uClibc, depending on the
  build options; I think it has an option that omits GNU regex...)

- Clarifying that the feature the option provides is not needed for
  conformance to POSIX but for extra GNU-version-compatible
  functionality beyond POSIX.

Would this be reasonable?

Rich