Allocate pathname buffers dynamically

Allocate pathname buffers dynamically

This change reworks numerous places which formerly used potentially
large on-stack buffers (of size AFSDIR_PATH_MAX) for constructing or
storing pathnames.  Instead, these buffers are now allocated from the
heap, either by using asprintf() to build a pathname in a correctly
sized buffer or, where necessary, using malloc() to allocate a buffer
of size AFSDIR_PATH_MAX.

A few occurrances of AFSDIR_PATH_MAX-sized buffers are not changed;
these are generally either globals or are contained within another
data structure that is already allocated on the heap.

[kaduk@mit.edu convert to cleanup-handler memory management where
appropriate]

Change-Id: Ib1986187a1c467e867d50280aaf1d8a86d9108c8
Reviewed-on: http://gerrit.openafs.org/9985
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>