checkcerts.sh: add some support for non-Linux systems

Date computations as performed in this script require the use of GNU date,
which is only available by default on Linux systems. With this check-in we
explicitly check for the existence of GNU date before continuing with the date
calculations.

Previous versions of this script relied on 'certtool' to print the expiration
dates but certtool isn't available by default on non-Linux systems either. The
previous check-in added support for using OpenSSL, retaining the old behavior
on non-Linux systems.

(Also a re-arrangement of the date warning logic)