Add framework for debug macros SLAP_OP(), slap_be_op(),
slap_bi_op() around LDAP-operation backend calls.

contrib/slapd-tools/wrap_slap_ops converts code to use them.

Code compiles as before by default.  #define USE_RS_ASSERT
enables debugging, $NO_RS_ASSERT tweaks it. See slapd/result.c.