+DW/29 Remove use of va_copy() which breaks pre-C99 systems. Duplicate the
+ result string, instead of calling string_vformat() twice with the same
+ arguments.
+
+DW/30 Allow TRUSTED_CONFIG_PREFIX_FILE only for Exim or CONFIGURE_OWNER, not
+ for other users. Others should always drop root privileges if they use
+ -C on the command line, even for a whitelisted configure file.