Change AFS*_LINUXnn_ENV to AFS*_LINUX_ENV

Change AFS*_LINUXnn_ENV to AFS*_LINUX_ENV

The minimum Linux kernel that is now supported is linux-2.6.18.  The
Linux versioned preprocessor macros AFS_*LINUXnn_ENV are no longer
needed to distinguish the different levels of Linux and can be merged
into just a single set of macros.

Perform a global change of _LINUX\d+_ENV to _LINUX_ENV. e.g.
   AFS_LINUX24_ENV       -> AFS_LINUX_ENV
   AFS_USR_LINUX24_ENV   -> AFS_USR_LINUX_ENV
   AFS_AMD64_LINUX20_ENV -> AFS_AMD64_LINUX_ENV

Replace the multiple definitions for the versioned 'AFS*_LINUXnn_ENV'
with just single non-version definitions 'AFS*_LINUX_ENV'.

Apart from replacing the now-redundant #define directives and tidying up
a few comments at the close of a preprocessor block to match their
current form, this commit was done using a mechanical change of the
variable names and did not reduce preprocessor statements that could now
be combined or eliminated.  Nor does this commit remove dead code.  A
follow-up commit (Cleanup AFS_*LINUX_ENV usage) will handle these
changes.

The updates should have no functional changes.

Change-Id: I71e32ca9818d28f82b4f23679868d1b9a62c44bd
Reviewed-on: https://gerrit.openafs.org/14387
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>