Set LAB to the branch name.

Modify alloctest for slab.  Also add a new SYS_nfree to report the
number of free pages.

devsw fixes

bug fix: exit() sometimes released a different parent lock than it acquired
usertests

and a usertest

if a program sbrk()'s to a non-page-boundary, don't free that page.
corresponding usertests.

add a regression test for it

This allows us to implement special files.

fileread/filewrite should validate major to avoid buffer overflows
or bogus function pointers.

Support running one test, by specifying the name as a command argument to usertests