Use octal escapes padded to three digits instead of hex escapes.
An octal escape is limited to three digits by definition, whereas
hex escape sequence are not limited, which causes problems if
a hex escape is followed by a (non escaped) possible hex character.

Escape double quotes and backslash. Use the simple escapes of
non printable characters too (optional, but looks nicer.)

This possibly could go in wsutil as another ws_escape method, but
the fix needs to be backported to 4.2 so let's not introduce a
new public function yet.

Fix #19735