Mention read_ahead tunables/sysctls.

MFC after:	1 month
Turns out -1 is a perfectly valid error number, ERESTART.  Remove useless
code written under assumption that it wasn't.

Sponsored by:	The FreeBSD Foundation
Allow SUBPAGE CODE field in MODE SENSE commands.
Add a few missing llvm/clang patches, update the other ones to be able
to apply with the same patch options onto a fresh upstream llvm/clang
3.4.1 checkout, and use approximately the same header tempate for them.

MFC after:	3 days