common/strtol.cc: fix the coverity warnings

common/strtol.cc: fix the coverity warnings

* promote the compared types properly to address the signed/unsigned
  comparison warnings. this also fixes the potential problems of
  slicing a compared type down to a "smaller" type before the
  comparison.
* check for the width of resulting type and shift bits caused by SI
  prefix to avoid the -Wshift-count-overflow warnings. this again
  is a potential issue, as shifting n bits of an integer of m bits
  width, where n >= m, leads to undefined behaviour.
* add a test for the 2nd fixed issue.

Signed-off-by: Kefu Chai <kchai@redhat.com>