missing: define kernel internal limit TASK_COMM_LEN in userspace too

We already use it at two places, and we are about to add one too.
Arbitrary literally hardcoded limits suck.

src/basic/missing.h

@@ -1407,4 +1407,11 @@ struct statx {
 #define AT_STATX_DONT_SYNC 0x4000
 #endif
 
+/* The maximum thread/process name length including trailing NUL byte. This mimics the kernel definition of the same
+ * name, which we need in userspace at various places but is not defined in userspace currently, neither under this
+ * name nor any other. */
+#ifndef TASK_COMM_LEN
+#define TASK_COMM_LEN 16
+#endif
+
 #include "missing_syscall.h"