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.

Reviewed by: kib
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D5589