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/fstatat.c

@@ -20,7 +20,9 @@
 #include "libc/calls/calls.h"
 #include "libc/calls/internal.h"
 #include "libc/errno.h"
+#include "libc/intrin/asan.internal.h"
 #include "libc/sysv/consts/at.h"
+#include "libc/sysv/errfuns.h"
 #include "libc/zipos/zipos.internal.h"
 
 /**
@@ -36,6 +38,7 @@
  */
 int fstatat(int dirfd, const char *path, struct stat *st, uint32_t flags) {
   struct ZiposUri zipname;
+  if (IsAsan() && (!st || !__asan_is_valid(st, sizeof(*st)))) return efault();
   if (weaken(__zipos_stat) && weaken(__zipos_parseuri)(path, &zipname) != -1) {
     return weaken(__zipos_stat)(&zipname, st);
   } else if (!IsWindows()) {