rx: Create AFS_ADAPT_PMTU and AFS_RXERRQ_ENV

rx: Create AFS_ADAPT_PMTU and AFS_RXERRQ_ENV

Currently we have the ADAPT_PMTU define, which turns on functionality
in Linux to detect PMTU-related ICMP errors for Rx. However, this is
really turning on two separate pieces of functionality: the PMTU
processing, and the processing for ICMP errors in general.

So split this out into two defines: AFS_ADAPT_PMTU, and
AFS_RXERRQ_ENV. The former is for processing PMTU discovery, and the
latter is for processing ICMP errors. Both of these are left disabled
due to issues in the error processing. Although PMTU discovery is the
only functionality which makes use of ICMP errors, this will change in
the future.

Change-Id: Ia334c68ce5eb3fa01c01a8a1c52a0e0a2e41b2c0
Reviewed-on: http://gerrit.openafs.org/7925
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>