Add octet-aligned/unaligned shift functions

The actual code is from OsmoBTS' tch.c by Harald Welte. Add unit tests,
doxygen annotation and extra memory safety check. Those functions are
used in several BTS implementations but seems generic enough to be
generally useful.

Change-Id: I2b1901c4161e8035f059585901dca593b661556d
4 files changed