Auto merge of #936 - bdrewery:freebsd-kevent-compat, r=alexcrichton

Link against kevent@FBSD_1.0 to fix ABI compat with FreeBSD12.

struct kevent was modified in FreeBSD12.  The @FBSD_1.0 symbol supports the old
structure ABI still.

This allows the `mio` crate tests to now pass on FreeBSD12.