1. Added a function cap_cnode_verify for cnodes lookup in cspace.
   Earlier cap_cnode_get was called to verify the cnode. It imposed
   extra restrictions on the user of libcap to release cnode lock.
   Now, cap_cnode_verify will itself take care of cnode lock.
2. Updated testcases to use cap_cnode_verify.

Signed-off-by: Pankaj Kumar <pankajk@cs.utah.edu>