[BusyBox] hush environment variables

[Larry Doolittle]

Evin discovered:
> Currently on my system (with gnu libc 2.1.3) doing export foo=blue
> doesn't work in CVS hush.
> The problem seems to be that the data passed to putenv is overwritten
> later, so the environment data no longer contains the variable when I
> try to use it.

Evin's analysis and patch look correct to me.

> Strangely things appear to work fine in lash even though the
> builtin_export funcions are basically the same in both lash and hush.

hush and lash handle argv[] very differently.  Either lash
leaks memory like a sieve, or you got lucky on your tests.
I suspect the latter.

> The following patch makes it work here.  Does it create a small memory
> leak?  Is there a better way?

A memory leak would show up if someone says:
$ export foo=blue
$ export foo=green
$ export foo=red
I doubt if putenv(3) is smart enough to free() the "foo=blue"
and "foo=green" strings.  lash and hush are certainly not that
smart.  I don't recommend spending effort on a fix.  The two
exceptions would be if:
 * it kills your application
 * you add local variable support at the same time

      - Larry