Fix environment variable assignment in init

The init process allows environment variables to be specified in
the rc script globally, via 'export', and for a specific process,
via the 'setenv' option.

However, the mechanism for assembling the environment simply
appended the new variable without checking whether it was already
defined, so it was not possible to overwrite an existing entry.
This patch fixes that behaviour.

Limitations and concerns:

The limit of a maximum number of 31 variables is unchanged.

Currently, most callers of "add_environment" do not check the return
value, but the function now logs the failure rather than failing
silently.

Change-Id: Ie9a68c37a0f55c5b40e904e695cd35514f67f480