process-util: be more careful in is_kernel_thread()

This reworks is_kernel_thread() a bit. Instead of checking whether
/proc/$pid/cmdline is entirely empty we now parse the 'flags' field from
/proc/$pid/stat and check the PF_KTHREAD flag, which directly encodes
whether something is a kernel thread.

Why all this? With current kernels userspace processes can set their
command line to empty too (through PR_SET_MM_ARG_START and friends), and
could potentially confuse us. Hence, let's use a more reliable way to
detect kernels like this.

src/basic/missing.h

@@ -1355,4 +1355,8 @@ struct fib_rule_uid_range {
 #define NS_GET_NSTYPE _IO(0xb7, 0x3)
 #endif
 
+#ifndef PF_KTHREAD
+#define PF_KTHREAD 0x00200000
+#endif
+
 #include "missing_syscall.h"