Simplify AIO initialization now that it is standard.

[jhb]

Summary

Simplify AIO initialization now that it is standard.

• Mark AIO system calls as STD and remove the helpers to dynamically register them.
• Use COMPAT6 for the old system calls with the older sigevent instead of an 'o' prefix.
• Simplify the POSIX configuration to note that AIO is always available.
• Handle AIO in the default VOP_PATHCONF instead of special casing it in the pathconf() system call. fpathconf() is still hackish.
• Remove freebsd32_aio_cancel() as it just called the native one directly.

I will remove the regen bits before comitting.

Test Plan

• Ran aio regression tests on amd64 for both amd64 and i386 binaries.