The buf/len members are now simply used in addition to the main
stream, without changing the handle kind.

Instead of handling shebang lines by adjusting the file pointer in
individual SAPIs, move the handling into the lexer, where this is
both a lot simpler and more robust. Whether the shebang should be
skipped is controlled by CG(skip_shebang) -- we might want to do
that in more cases.

This fixed bugs #60677 and #78066.

Uses system allocator when backing up settings ... let's avoid
unnecessary confusion.

It seems that this code has a peculiar interpretation of "len",
where it actually points to the last character, not one past it.
So we need +1 here for that extra char and another +1 for the
terminating null byte.

* PHP-7.3:
  Fix #77805 phpdbg build fails when readline is shared

* PHP-7.2:
  Fix #77805 phpdbg build fails when readline is shared