Added checks for BSD-compliant protocol constants.
Fixed the checks for a couple of Linux-specific constants so they work correctly (they included the wrong header).
Added a check for the Linux-specific constant IP_PMTUDISC_DO; this'll be used with IP_MTU_DISCOVER in a future patch to make Linux sockets match BSD/OSX/Windows behavior.
Merge pull request #556 from jack-pappas/ipproto-patch

Added checks for BSD-compliant protocol constants.