Namespaced includes of libraries #358

This is part of tackling of issue #200, of which the progress can be followed in #349.

I think the public header files (as are installed on the system and used while compiling and linking executables and other libraries against the base one[s]) should be namespaced - so in the end these would be included as namespace/header.h|hh, for example hal/hal.h. I already implemented it for the base libraries as part of #349.

The main ideas behind it are:

• It seems to be a standard practise with many projects using the workspace/superbuild (etc) approach are using
• There are actually headers named the same way in the current Machinekit-HAL's sourcebase - namely the hal.h part of the HAL and the hal.h part of the XHC WBH04B-6 module. Machinekit-HAL should force only the uniqueness of the main names on the module creators, not how they name every file - in other words, the hal.h file should be fine without risk of any clashes
• Using namespaces will also automatically signal to anybody who is reading the code on which other parts the source depends. (Without reading the build recipes.)