gpt-auto-generator: honour read-only and no-auto flag from GPT data

Similar to the read-only and no-automount flags of Microsoft Basic Data
Partitions, introduce our own flags. We map them to the same flag bits
as Microsoft's, to keep things simple.

src/shared/gpt.h

@@ -39,3 +39,10 @@
 #elif defined(__i386__)
 #  define GPT_ROOT_NATIVE GPT_ROOT_X86
 #endif
+
+/* Flags we recognize on the root, swap, home and srv partitions when
+ * doing auto-discovery. These happen to be identical to what
+ * Microsoft defines for its own Basic Data Partitions, but that's
+ * just because we saw no point in defining any other values here. */
+#define GPT_FLAG_READ_ONLY (1ULL << 60)
+#define GPT_FLAG_NO_AUTO (1ULL << 63)