Allow end users to build source packages, sign packages with their own
GnuPG key, and perform other interactions with `dpkg-buildpackage`.

```
RC=0 stuartl@rikishi ~/vrt/projects/metermaster/libs/libmbus $ ./build-deb.sh --foo --bar -mokey 7 --skip-binary --hello --
Usage: ./build-deb.sh [--build-source-pkg] [--skip-binary]
          [{--sign-key|-l} <GPG KEY ID>]
          [-- <dpkg-buildpackage options>]
Unrecognised argument --foo
RC=1 stuartl@rikishi ~/vrt/projects/metermaster/libs/libmbus $ ./build-deb.sh -h
Usage: ./build-deb.sh [--build-source-pkg] [--skip-binary]
          [{--sign-key|-l} <GPG KEY ID>]
          [-- <dpkg-buildpackage options>]
RC=0 stuartl@rikishi ~/vrt/projects/metermaster/libs/libmbus $ ./build-deb.sh -?
Usage: ./build-deb.sh [--build-source-pkg] [--skip-binary]
          [{--sign-key|-l} <GPG KEY ID>]
          [-- <dpkg-buildpackage options>]
RC=0 stuartl@rikishi ~/vrt/projects/metermaster/libs/libmbus $ ./build-deb.sh --help
Usage: ./build-deb.sh [--build-source-pkg] [--skip-binary]
          [{--sign-key|-l} <GPG KEY ID>]
          [-- <dpkg-buildpackage options>]
```