Link checker: Allow configurable User Agent string
Credit to Bill Erickson for noticing that tests were resulting in an
inordinate number of 403 Forbidden responses, which turned out to be
due to discrimination by sites against a libwww/* user agent string.
We now use "Evergreen <version> Link Checker" by default, and it's
configurable in opensrf.xml (grep for user_agent).
Signed-off-by: Lebbeous Fogle-Weekley <lebbeous@esilibrary.com> Signed-off-by: Mike Rylander <mrylander@gmail.com>