SysV IPC APIs missing on OpenBSD

[gnzlbg]

See this issue: gnzlbg/slice_deque#77 (comment)

[landryb]

cf landryb/libc@8c688c2 for a quick wip. @semarie ?

[semarie]

@landryb thanks for the wip. I will use it as base for the PR.

I will have few fixes (but I will take care of that):

• sys/shm.h header is need in libc-test/build.rs for proper testsuite run
• shm_segsz is int and not a size_t on OpenBSD (hey, not posix compliant. I hope others crates doesn't rely on that)
• struct shmid_ds is incomplete
• few constants mismatches