Add envz_remove to the libc manual

Add envz_remove to the libc manual

I was told that Ma Shimao submitted a patch to add envz_remove to the
libc manual, but the patch could not be accepted since he does not
have a copyright assignment in place.  I have been woefully behind on
libc-alpha recently and have not seen the patch or the discussion
thread.  I have also not read the man page for envz_remove, so
Alexandre Oliva asked me if I could write this independently and post
a patch.  The patch below is the result of the same - I have written
it based on the implementation in string/envz.c and Alex told me via
email that the function is AS, AC and MT-safe like envz_strip.

I assume Alex and Carlos cannot review this since they have been
tainted by the original patch (I haven't even tried to look for a link
to it since I don't want to be tainted) so someone else will have to
review this.  If there are no reviewers till the end of the week, I
will commit this since I believe there is a chance that there are no
other reviewers who haven't read that thread.

	* manual/string.texi (Envz Functions): Add envz_remove.