[PTFM-4820] Various dxcpp changes (may break existing code):
  - All handler classes, and helper functions, etc are in namespace dx now
  - g_* are no longer global variables, but rather created on first use (present in dx::config)
  - Configurable user-agent string in dxcpp (by default dx-toolkit version is used)
  - UA & dx-verify-file have been fixed to use params in dx::config, instead of g_*
  - Various other small fixes