Refactor: based, libcib: Replace cib_op_t * with cib_op_t

Functions and pointers to functions are treated identically in C. This
avoids a strange const hell in an upcoming commit, in which we retrieve
cib_operation_t:fn directly in the caller instead of calling
cib_op_func().

Ref T658

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>