libipsec: make const-correct

• add const to the appropriate places in the libipsec public API and the relevant internal functions needed to support that.

• replace caddr_t with c_caddr_t in ipsec_dump_policy()

• update the ipsec_dump_policy manpage to use c_caddr_t (this manpage was already wrong as it had "char *" instead of caddr_t previously).

While here, update pfkeyv2.h to not cast away const in the PFKEY_*()
macros.

This should not cause any ABI changes as the actual types have not
changed.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1099