coding guidelines: comply with MISRA C:2012 Rule 7.2

MISRA C:2012 Rule 7.2 (A `u' or `U' suffix shall be applied to all
integer constants that are represented in an unsigned type)

Added missing `U' suffixes in constants that are involved in the
analyzed build, plus a few more not to introduce inconsistencies
with respect to nearby constants that are either unused in the
build (but implicitly unsigned) or are used and are immediately
converted to unsigned.

Signed-off-by: Abramo Bagnara <abramo.bagnara@bugseng.com>