Minor touchup for tsc-compat

It wasn't compiling with some Linux programs.  Slightly odd, since I've
used this with gcc on Linux in the past.  Perhaps those programs also
defined FALSE.
Default signal handlers are sigactions

Instead of the weaker sighandlers.  This is useful so we have access to
the faulting context, such as in default_core_handler.

Incidentally, you may want to point a breakpoint in that handler if you
want to kfunc print_vmrs to debug the page fault.
Fixes memory leak with syscall strings

Yield and exec don't return, so we would never free those blobs.