Factor out VG_(exit_now) to contain the syscall incantation to terminate

Factor out VG_(exit_now) to contain the syscall incantation to terminate
the process. Make ML_(am_exit) and VG_(exit) use it, thereby avoiding
double maintenance.
Introduce libcbase_assert macro and use it in VG_(strncpy_safely) to
document the case that function cannot handle.
Add stub functions to memcheck/tests/unit_libcbase.c to satisfy new
dependencies.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14185