Make redbean StoreAsset() work better

- Better UBSAN error messages
- POSIX Advisory Locks polyfills
- Move redbean manual to /.help.txt
- System call memory safety in ASAN mode
- Character classification now does UNICODE

libc/calls/flock.c

@@ -23,6 +23,8 @@
 /**
  * Acquires lock on file.
  *
+ * Please note multiple file descriptors means multiple locks.
+ *
  * @param op can have LOCK_{SH,EX,NB,UN} for shared, exclusive,
  *     non-blocking, and unlocking
  * @return 0 on success, or -1 w/ errno