Move some pthread functions to signal.h.

POSIX specifies that pthread_kill(3) and pthread_sigmask(3) are
supposed to live in signal.h rather than pthread.h.

Since signal.h now needs pthread_t and pthread_attr_t, I've moved
those defintions into include/machine/pthread_types.h to keep the
namespace clean. I also sorted some includes. The combination of these
two things seems to have exploded into a cascade of missing includes,
so this patch also cleans up all those.

Change-Id: Icfa92a39432fe83f542a797e5a113289d7e4ad0c
diff --git a/libc/private/kernel_sigset_t.h b/libc/private/kernel_sigset_t.h
index b2d6386..9415fcf 100644
--- a/libc/private/kernel_sigset_t.h
+++ b/libc/private/kernel_sigset_t.h
@@ -17,6 +17,8 @@
 #ifndef LIBC_PRIVATE_KERNEL_SIGSET_T_H_
 #define LIBC_PRIVATE_KERNEL_SIGSET_T_H_
 
+#include <signal.h>
+
 // Our sigset_t is wrong for ARM and x86. It's 32-bit but the kernel expects 64 bits.
 // This means we can't support real-time signals correctly until we can change the ABI.
 // In the meantime, we can use this union to pass an appropriately-sized block of memory