sync-mail: Warn user as part of prehook

If no hook should be run before syncing (or a user-defined hook) then
the warning should fall under that and not be run as part of it.

Additionally takes care of multiple decryption requirements in a row.

office/.local/bin/sync-mail

@@ -47,6 +47,7 @@ prehook() {
         eval "$MBSYNC_PRE"
         return 0
     fi
+    checkwarnuser
     imapfilter -c "${XDG_CONFIG_HOME:-$HOME/.config}/imapfilter/config.lua"
 }
 
@@ -142,7 +143,6 @@ elif [ -n "$1" ]; then
 fi
 
 main() {
-    checkwarnuser
     enablegpgagent
     prehook