correct issues pointed out by reviewers

* debian/changelog: Correct s/ustable/unstable/
* debian/control: Set python3-plainbox-doc Priority to extra
* debian/control: Remove leading capitalization and trailing dot from the
  description field.
* debian/control: Build-depend on help2man
* debian/patches: Add 01-add-main-module and 02-executable-laucher1 patches
  so that `python3 -m plainbox` and `python3 -m
  plainbox.impl.secure.launcher1` can be used to invoke plainbox and
  plainbox-trusted-launcher-1
* debian/patches: Add 03-unvendorize so remove bundled copies of
  python3-funcsigs and python3-mock that are not needed in Debian
* debian/rules: Remove plainbox/vendor/funcsigs directory on clean
* debian/rules: Generate plainbox.1 and plainbox-trusted-launcher-1 with
  help2man at build time and remove them at clean time.
* debian/rules: Don't use dh_installdocs --link-doc to prevent every package
  from depending on python3-plainbox-doc
* debian/plainbox.manpages, debian/python3-plainbox.manpages: Install the
  generated manual pages