Set default User-Agent to something that doesn't mention libwww-perl

It appears that both the open-source and proprietary rulesets for
ModSecurity default to blacklisting requests that say they are
from libwww-perl, presumably because some script kiddies use libwww-perl
and are too inept to set a User-Agent that is "too big to blacklist",
like Chrome or the iPhone browser or something. This seems doomed to
failure but whatever.