SSL.Constants: Encrypt-then-MAC is now RFC 7366.

Updated reference. Also adds a note about the "Prohibit RC4" draft.
Parser.HTML: Reduce pushing of empty strings.

The empty string was pushed before and after every quoted attribute.

This delays pushing of the empty string until after the attribute
value has been parsed and it was empty. This avoids having to call
f_add() for every quoted attribute to concatenate the strings.
Parser.HTML: Added mode quote_stapling().

This is a compatibility mode with the Pike 7.8 and earlier Parser.HTML.

Do not use if you can avoid it.
signal_handler: don't "randomly" change errno in receive_sigchild()