various updates to checkcerts script

- add support for 'openssl'
- parse expiration date, failing if expired or if expires within 30 days
- warn at 60
remove debug text
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)
checkcerts.sh: cross platform support on *NIX

- function to convert dates to Julian to perform date calculation if GNU date is
  not present.