kernel/thread: restore signal mask just before returning to userland

* otherwise the signal to be handled might be blocked. fixes #15193
* also remove automatic syscall restart on _kern_select, to match Linux and
BSDs behavior: this fixes parallel build with newer gnu make, which happens
to use pselect.
* also remove automatic syscall restart on _kern_poll.

from https://man7.org/linux/man-pages/man7/signal.7.html
"The following interfaces are never restarted after being
       interrupted by a signal handler, regardless of the use of
       SA_RESTART; they always fail with the error EINTR when
       interrupted by a signal handler: ...
	select(2), and pselect(2)."
from https://notes.shichao.io/unp/ch6/
"Berkeley-derived kernels never automatically restart select."

Change-Id: I7f86d221eae1ad93d8a308a75581d2c30a369c9e
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3627
Reviewed-by: Adrien Destugues <[email protected]>
Reviewed-by: waddlesplash <[email protected]>

headers/private/kernel/thread_types.h

@@ -455,6 +455,10 @@ struct Thread : TeamThreadIteratorEntry<thread_id>, KernelReferenceable {
 		// non-0 after a return from _user_sigsuspend(), containing the inverted
 		// original signal mask, reset in handle_signals(); only accessed by
 		// this thread
+	sigset_t		old_sig_block_mask;
+		// the old sig_block_mask to be restored when returning to userland
+		// when THREAD_FLAGS_OLD_SIGMASK is set
+
 	ucontext_t*		user_signal_context;	// only accessed by this thread
 	addr_t			signal_stack_base;		// only accessed by this thread
 	size_t			signal_stack_size;		// only accessed by this thread
@@ -844,5 +848,7 @@ using BKernel::ProcessGroupList;
 #define	THREAD_FLAGS_COMPAT_MODE			0x2000
 	// the thread runs a compatibility mode (for instance IA32 on x86_64).
 #endif
+#define	THREAD_FLAGS_OLD_SIGMASK			0x4000
+	// the thread has an old sigmask to be restored
 
 #endif	/* _KERNEL_THREAD_TYPES_H */