Fix build on OS X

* blkgetsize_stubs.c is from mirage/mirage-block-unix
* O_DIRECT is not present on OS X; use the closest analog
* setsockopt level SOL_TCP is IPPROTO_TCP on OS X
* setsockopt TCP_KEEPIDLE is Linux-specific

Signed-off-by: David Scott <dave.scott@citrix.com>