Provide fallback code for non-portable clearenv(3)

Provide fallback code for non-portable clearenv(3)

clearenv(3) is a GNU extension that was rejected twice by POSIX,
without an easy, portable alternative.

As the dovecot project notes,
"""
environ = NULL crashes on OSX - and OpenBSD < 6.0
*environ = NULL doesn't work on FreeBSD 7.0
environ = emptyenv doesn't work on Haiku OS
environ = calloc should work everywhere
"""
(source: http://hg.dovecot.org/dovecot-2.0/file/48f90e7e92dc/src/lib/env-util.c)

Bug: https://bugzilla.samba.org/show_bug.cgi?id=11864
Signed-off-by: Jérémie Courrèges-Anglas <jca@wxcvbn.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Fri Apr 29 00:12:02 CEST 2016 on sn-devel-144