chore(linux): Fix lintian warnings

- add rudimentary man page for keyman-system-service
- remove hashbang for bash completion files
- reword description for keyman-system-service

This fixes these lintian warnings:
keyman-system-service: description-is-pkg-name Keyman system service
keyman: bash-completion-with-hashbang /usr/bin/env bash
        [usr/share/bash-completion/completions/km-config:1]
keyman-system-service: no-manual-page
        [usr/libexec/keyman-system-service]