Various updates in verify_sdp.sh v5.3.0:
* Passes shellcheck v.0.6.0 check; which drove various minor
code changes, e.g. using bash built-in 'command -v' rather than
system utility 'which', and others.
* Improved usage of mktemp util to use P4TMP in a template, to
avoid SELinux issues.
* Fixed issue where early calls to bail() resulted in output
not being displayed.
* Bullet-proofed for scenarios where script is called by the
wrong user, or called by correct user after first calling by
wrong user and log ownership is wrong, or user can't write log
file, or P4TMP not defined, etc.
* Clarified TODO comments.