Reorganize gatekeeper to reflect processing phases

Reorganize gatekeeper to reflect processing phases

Prior to committing, this was validated with:
    (DIFF='git diff --cached';
     comm -3 <($DIFF | grep ^- | sed -e 's/^-//' | sort) \
             <($DIFF | grep ^+ | sed -e 's/^+//' | sort) )

The output shows only blank lines, diff headers, POD structure, and the
addition of a forward declaration for verify_clearsigned_message as a
temporary measure ahead of a call that will eventually be removed, after
all lines are sorted and paired for analysis.  To replicate, change the
"git diff" command to compare this commit with its parent.