opr: constify various string functions and mark them AFS_NONNULL()

opr: constify various string functions and mark them AFS_NONNULL()

All of these string functions require at least one non-null argument.
Mark them as AFS_NONNULL() so that the compiler and static checker can
find erroneous uses.  The "source" arguments of lcstring and ucstring
can be const, so do so.  (This doesn't affect anything in the tree
right now.)  While here, note a few unfixed issues with these interfaces.)

Change-Id: If2a8dd4d617795560e92c09ee604780f90edce6a
Reviewed-on: http://gerrit.openafs.org/7804
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>