thread-mainloop: keep SIGSYS unblocked if currently trapped

thread-mainloop: keep SIGSYS unblocked if currently trapped

Seccomp-BPF uses SIGSYS signal to trigger
the trap handler attached to sys_open.
If the signal is blocked then the kernel kills
the process whenever pulse audio calls 'open'.
The result backtrace is terminating in sys_open.

That's why it is required to keep SIGSYS unblocked
if it is currently unblocked and trapped.

This patch allows to have pulse audio working
in the Chromium sandbox.

Signed-off-by: Julien Isorce <j.isorce@samsung.com>
Signed-off-by: Arun Raghavan <git@arunraghavan.net>