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/struct/flock.h

@@ -2,12 +2,13 @@
 #define COSMOPOLITAN_LIBC_CALLS_STRUCT_FLOCK_H_
 #if !(__ASSEMBLER__ + __LINKER__ + 0)
 
-struct flock {
-  short l_type;
-  short l_whence;
-  int64_t l_start;
-  int64_t l_len;
-  int l_pid;
+struct flock {      /* cosmopolitan abi */
+  int16_t l_type;   /* F_RDLCK, F_WRLCK, F_UNLCK */
+  int16_t l_whence; /* SEEK_SET, SEEK_CUR, SEEK_END */
+  int64_t l_start;  /* starting offset */
+  int64_t l_len;    /* 0 means until end of file */
+  int32_t l_pid;    /* lock owner */
+  int32_t l_sysid;  /* remote system id or zero for local */
 };
 
 #endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */