unicorn 5.4.0

Rack hijack support improves as the app code can capture and use
the Rack `env' privately without copying it (to avoid clobbering
by another client).  Thanks to Sam Saffron for reporting and
testing this new feature:
  https://bogomips.org/unicorn-public/CAAtdryPG3nLuyo0jxfYW1YHu1Q+ZpkLkd4KdWC8vA46B5haZxw@mail.gmail.com/T/

We also now support $DEBUG being set by the Rack app (instead of
relying on the "-d" CLI switch).  Thanks to James P Robinson Jr
for reporting this bug:
  https://bogomips.org/unicorn-public/D6324CB4.7BC3E%25james.robinson3@cigna.com/T/
  (Coincidentally, this fix will be irrelevant for Ruby 2.5
   which requires 'pp' by default)

There's a few minor test cleanups and documentation updates, too.

All commits since v5.3.1 (2017-10-03):

    reduce method calls with String#start_with?
    require 'pp' if $DEBUG is set by Rack app
    avoid reusing env on hijack
    tests: cleanup some unused variable warnings
    ISSUES: add a note about Debian BTS interopability

Roughly all mailing discussions since the last release:

  https://bogomips.org/unicorn-public/?q=d:20171004..20171223