Windows: avoid use of cm_buf for MPs and Symlinks

Windows: avoid use of cm_buf for MPs and Symlinks

In the Windows cache manager, the symlink and mount point
target strings are stored in the cm_scache_t mountPointString
and are not accessed out of the cm_buf_t for offset zero
except when populating the mountPointString.  As a result,
every mountpoint and symlink object that is read into the cache
wastes a cm_buf_t which could otherwise be used to store
additional file or directory data.

Add cm_GetData() function which is similar to cm_GetBuffer()
except that it reads data from the file server into an arbitray
memory location instead of a cm_buf_t object.  Use cm_GetData()
to read directly into the cm_scache_t object.

In addition, further optimize the communication with the
file server by using cm_GetData() to perform a RXAFS_FetchData
RPC to obtain both the target string and the status information
instead of RXAFS_FetchStatus which only returns the status
information in cases where there are no outstanding callback
registrations on the object.  RXAFS_FetchStatus is still used
when a callback is active in order to obtain access permissions
for new users.

Change-Id: I4d797479624f2e29121b16d3aa381296a57aeaa6
Reviewed-on: http://gerrit.openafs.org/4111
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>