libafs: Implement unixuser RW locks

libafs: Implement unixuser RW locks

Currently code dealing with changing unixuser structs does not obtain
any locks protecting the contents of the unixuser struct, though some
functions like afs_GetUser have a parameter indicating what type of
lock should be obtained. This can result in the token data for a user
being changed at the same time another thread tries to use the token
data.

To ensure mutual exclusion of such operations, add a lock field to the
unixuser struct, and actually lock it according to the intentions of
the relevant code.

Change-Id: Idd66d72f716b7e7dc08faa31ae43e9a23639bae3
Reviewed-on: http://gerrit.openafs.org/4636
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>