Implement a cheritest 'revoke_fd' test case that invokes cheri_fd_revoke
to revoke a delegated cheri_fd object (appears to work!).

Fix argv[] indexing for cheri_fd test cases so they work properly when
more than one is used at a time.
Include <string.h> since strlen() is now used in cheritest-helper.
Make cheri_fd_revoke() and cheri_fd_destroy() take an object-capability
reference (cheri_object) rather than a pointer to an object-capability
reference.