Added checks for BSD-compliant protocol constants.

Added checks for BSD-compliant protocol constants. These will be needed for some future improvements to Mono's socket-handling code.

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.

These changes are released under the MIT/X11 license.