Obsoleting NSPRPUB_PRE_4_2_CLIENT_BRANCH

[Kai Engert]

After discussing with Wan-Teh Chang, in order to simplify the
maintenance of NSPR, we'd like to obsolete the
NSPRPUB_PRE_4_2_CLIENT_BRANCH.

As of today, NSPR actively uses:
- NSPR_4_6_BRANCH
- HEAD
- NSPRPUB_PRE_4_2_CLIENT_BRANCH

Releases based on NSPR_4_6_BRANCH are used by stable Mozilla branches.

NSPRPUB_PRE_4_2_CLIENT_BRANCH is used by Mozilla trunk.

NSPRPUB_PRE_4_2_CLIENT_BRANCH got introduced at some point in the past
when there was a need to have a separate snapshot for Mozilla.

As of today, HEAD and NSPRPUB_PRE_4_2_CLIENT_BRANCH are mostly in synch.
Whenever checkins are made to HEAD, usually the patch will land on
NSPRPUB_PRE_4_2_CLIENT_BRANCH, too.

We'd like to avoid having to check in NSPR development patches to two
branches.

However, Mozilla trunk should not pull NSPR HEAD directly. Instead, we
propose to produce static tags of NSPR's HEAD as needed and change
NSPR_CO_TAG in mozilla/client.mk to pick up new tags.

(Mozilla on stable branches shall continue to use NSPR releases based on
an NSPR stable branch, not NSPR HEAD.)

Please let us know if you have objections to this proposal.

Thanks,
Kai