- Add support for the ucontext system calls (getcontext, setcontext,

swapcontext, and makecontext).
- Fix VM to not erroneously think the stack segment and data segment have
  collided when a user-space thread invokes brk().
- Add test51 to test ucontext functionality.
- Add man pages for ucontext system calls.

test/run

@@ -13,13 +13,13 @@ badones=			# list of tests that failed
 
 # Print test welcome message
 clr
-echo "Running POSIX compliance test suite. There are 54 tests in total."
+echo "Running POSIX compliance test suite. There are 56 tests in total."
 echo " "
 
 # Run all the tests, keeping track of who failed.
 for i in  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 \
          21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 \
-         41 42 43 44 45 45-gcc 46 47 48 49 49-gcc 50 sh1.sh sh2.sh
+         41 42 43 44 45 45-gcc 46 47 48 49 49-gcc 50 51 51-gcc sh1.sh sh2.sh
 do
    if [ -x ./test$i ]
    then