Split into multiple files

For multiple reasons the program is now split:
- Make future porting to OpenBSD easier
- Assign header includes to individiual functions
- Make future program extensions easier
- Recompile only changed parts

kernel_release.c

@@ -0,0 +1,16 @@
+#include <sys/utsname.h>
+#include <stdio.h>
+
+#include "util.h"
+
+const char *
+kernel_release(void)
+{
+	struct utsname udata;
+
+	if (uname(&udata) < 0) {
+		return NULL;
+	}
+
+	return bprintf("%s", udata.release);
+}