Various clean ups in the xtimer implementation to prepare for some functional enhancements.

• Two commits are taken from @kaspar030's sys: make xtimer isr-safe #5428, with one two fixups added by me for fixing some minor left overs in the configuration.
• The xtimer.h header was getting a bit long and unwieldy so I decided to split it into one API declaration header and one implementation header for defining the inline functions.
• The _lltimer functions are available from the xtimer.h header so they have to be prefixed in order to keep the namespace conventions. There is still an underscore prefix used to indicate that these functions are meant to be private and should not be called from user applications.
• Renamed XTIMER -> XTIMER_DEV to make the macro less generic and more describing.
• Renamed the type timer_callback_t -> xtimer_callback_t
• Delete an old test xtimer_shift_on_compare which has no use any more.