Support the NTP SHM refclock on FreeBSD - #423
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The SHM writer was gated to linux and darwin because x/sys/unix provides its SysvShm* wrappers only for those platforms. The syscall numbers are defined for freebsd, so route the shmget/shmat/shmdt/ shmctl calls through thin per-platform wrappers: linux and darwin delegate to x/sys/unix as before, and a freebsd file issues the syscalls directly, carrying the IPC_CREAT and IPC_RMID values from sys/ipc.h that x/sys/unix also lacks there. The freebsd attach returns a slice of the expectedSize bytes this package uses, which shmget has already checked the segment can hold. The shmTime layout is generated from the shared types.go input; time_t is 64-bit on freebsd amd64 and arm64, so the layout and expectedSize 0x60 match darwin. Verified on FreeBSD 15.1 against the base ntpd (4.2.8p18), which ships with the SHM refclock compiled in: satpulsed publishing serial PPS samples to segment 2 shows up in ntpq -p with sub-ms offset and about 15 us jitter.
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The SHM writer was gated to linux and darwin because x/sys/unix provides its SysvShm* wrappers only for those platforms. The syscall numbers are defined for freebsd, so this routes the shmget/shmat/shmdt/shmctl calls through thin per-platform wrappers: linux and darwin delegate to x/sys/unix as before, and a freebsd file issues the syscalls directly, carrying the IPC_CREAT and IPC_RMID values from sys/ipc.h that x/sys/unix also lacks there. The shmTime layout is generated from the shared types.go input; time_t is 64-bit on freebsd amd64 and arm64, so the layout and expectedSize 0x60 match darwin.
Verified on FreeBSD 15.1 against the base ntpd (4.2.8p18), which ships with the SHM refclock compiled in: satpulsed publishing serial PPS samples to segment 2 shows up in ntpq -p with sub-ms offset and single-digit-microsecond jitter.
Based on #422, which it merges after.
Part of #426.