libipsec: make const-correct

[llfw]

• 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.

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this came up during my work on the traceroute merge (and will make that easier) but should fixed regardless of that.