fix a bug that caused time-based rekeys to happen too frequently.

rename the structure internals to id32_* in anticipation of an
idgen16() that might come in the future.

implement automatic time-based rekeying (every 10 minutes); ok deraadt@

Introduce a facility to generate unpredictable 32 bit numbers with
near maximal (2^32) cycle times. These are useful for network
IDs in cases where there are negative consequences to ID prediction
an

Introduce a facility to generate unpredictable 32 bit numbers with
near maximal (2^32) cycle times. These are useful for network
IDs in cases where there are negative consequences to ID prediction
and/or reuse.

Use the idgen32() functions to generate IPv6 IDs and NFS client/server
XIDs.

Pseudorandom permutation code in crypto/idgen.c based on public
domain skip32.c from Greg Rose.

feedback & ok thib@ deraadt@

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