aklog: Add replacement setenv/unsetenv

aklog: Add replacement setenv/unsetenv

aklog makes use of the setenv and unsetenv functions, which do not
exist (at least) on HP-UX earlier than around 11i v3, and do not exist
on Solaris earlier than Solaris 10. Add replacement functions for
setenv and unsetenv when they are not present. Note that these
implementations are copied from libroken, and setenv was modified to
not use asprintf.

This is 1.6-specific. On the master branch, libroken takes care of
these for us. On the master branch, setenv and unsetenv from libroken
were added in 70e8451acd0426024c152073e53bc6606e0189e1.

Change-Id: I35546f1add7f4f87c6ffc484059057825887499f
Reviewed-on: http://gerrit.openafs.org/6376
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementix.org>