HRW: Add support for elif, in if-elif-else

[zwoop]

This adds an elif clause to the conditions. E.g.

cond %{SEND_RESPONSE_HDR_HOOK} [AND]
cond %{CLIENT-HEADER:X-Foo} ="foo"
   set-header X-Resp "foo"
elif
    cond %{CLIENT-HEADER:X-Foo} ="bar"
        set-header X-Resp "bar"
elif
    cond %{CLIENT-HEADER:X-Foo} ="baz"
        set-header X-Resp "baz"
else
   set-header X-Resp "else"

In addition, this cleans up the handling of the existing "else" clause, and I ran clang-tidy over the two files I was editing.

Co-author: Miles L.

[zwoop]

[approve ci osx]

[zwoop]

[approve ci]