Allow GetSomeSpace_r to select an optimal host

Allow GetSomeSpace_r to select an optimal host

Previously GetSomeSpace_r would never find an 'ideal' host for which to
clear callbacks, since lih0_r and lih1_r required a non-NULL rock to do
anything. Remove the requirement for the passed-in host rock to be
non-NULL, and make lih*_r more threadsafe, by passing in a parameter
struct for the rock.

Also attempt to make the GSS_r code a bit more clear with some
descriptive variable names and such.

FIXES 126451

Change-Id: Id754f4fa4b830896c5b03fc7ba0906950991a3b7
Reviewed-on: http://gerrit.openafs.org/1317
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>