sync: Cache knowledge of kernel uapi version

Previously all libsync calls would try first the modern/mainline uapi
and if that failed try the legacy uapi, or vice versa. This is
inefficient, and confusing when looking at strace. With this change,
after the first successful syscall, libsync know's what uapi version
the kernel supports, and will only try that version in the future.

Test: sync-unit-tests on bullhead
Change-Id: I8b5de0194da0cfc6c080c0180318e16bb673d3c9