userok.c: Fix fixed-size on-stack path buffers

Several functions in src/auth/userok.c construct pathnames in fixed
size buffers on their stacks.  Those buffers are simultaneously too
small for the purpose for which they are used and too large to be
placed on the stack.  This change replaces these fixed-size buffers
with dynamically-allocated buffers which are either exactly the right
size (due to asprintf) or have size AFSDIR_PATH_MAX.

FIXES 130719

Change-Id: I49b1c03d4d3525b87e155eb2d6eedf4b199a33c5
Reviewed-on: http://gerrit.openafs.org/9986
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>