[LITMUS^RT] P-FP plugin together with PCP

[Sven Dziadek]

Hi all,
I have ported the P-FP (fixed-priority) plugin from Björn's dissertation 
(branch bbb-diss) to the current version of Litmus.
Additionally I changed it so that PCP is available to user space.

Björn and I also made a few cosmetic changes concerning the priorities:
We now have explicit "LITMUS_HIGHEST_PRIORITY" and 
"LITMUS_LOWEST_PRIORITY" together with macros for comparison so that 
there cannot arise any confusion.

Please have a look at prop/P-FP (in litmus-rt and liblitmus) and 
complain if you have any objections.

Otherwise this should please be merged into staging.

Thanks,
Sven