ptrace: add ability to get/set signal-blocked mask

ptrace: add ability to get/set signal-blocked mask

crtools uses a parasite code for dumping processes.  The parasite code is
injected into a process with help PTRACE_SEIZE.

Currently crtools blocks signals from a parasite code.  If a process has
pending signals, crtools wait while a process handles these signals.

This method is not suitable for stopped tasks.  A stopped task can have a
few pending signals, when we will try to execute a parasite code, we will
need to drop SIGSTOP, but all other signals must remain pending, because a
state of processes must not be changed during checkpointing.

This patch adds two ptrace commands to set/get signal-blocked mask.

I think gdb can use this commands too.

[[email protected]: be consistent with brace layout]
Signed-off-by: Andrey Vagin <[email protected]>
Reviewed-by: Oleg Nesterov <[email protected]>
Cc: Roland McGrath <[email protected]>
Cc: Michael Kerrisk <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Cc: Cyrill Gorcunov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>