GPG key rollover.

This commit adds the new ids and fingerprints in the keys appendix of
the manual, and moves the old ones down into the historic-keys
section. I've tweaked a few pieces of wording for ongoing use, so that
they don't imply a specific number of past key rollovers.

The -pgpfp option in all the tools now shows the new Master Key
fingerprint and the previous (2015) one. I've adjusted all the uses of
the #defines in putty.h so that future rollovers should only have to
modify the #defines themselves.

Most importantly, sign.sh bakes in the ids of the current release and
snapshot keys, so that snapshots will automatically be signed with the
new snapshot key and the -r option will invoke the new release key.

sign.sh

@@ -9,14 +9,14 @@
 
 set -e
 
-keyname=EEF20295D15F7E8A
+keyname=38BA7229B7588FD1
 preliminary=false
 
 while :; do
     case "$1" in
         -r)
             shift
-            keyname=9DFE2648B43434E4
+            keyname=6289A25F4AE8DA82
             ;;
         -p)
             shift