[PATCH 0/2] sigaltstack: remove EPERM check

The following patches add the self-test for
sigaltstack(SS_DISABLE) inside the signal handler,
and remove the EPERM check in a sigaltstack()
syscall.
This is needed to make sigaltstack() compatible with
swapcontext(): before using swapcontext() inside
the sighandler, the sigaltstack should be disabled, or
the stack will be corrupted by the subsequent signals.