fs: Abstract out code to get the last component

fs: Abstract out code to get the last component

fs.c contained two identical copies of code to parse a path, resolve
any symbolic links in that path, and return the directory and basename.

Abstract this code out into a single function, rather than maintaining
two separate copies of it.

Change-Id: Id9fc835506f90bae2fc1f0db18c640bf28f0d39d
Reviewed-on: http://gerrit.openafs.org/4462
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>