B"H add relative url allowed option, to match better the spec

before was only considering "/" at beginning to be relative URLs,
but rela-urls can start with other things (".", "../"),
and they can even be the empty string!
B"H change url relativeness depends on last url, add url tests

fixes -p for 1st url
add tests for multiple URLs
B"H forgot to add longurl.py to last commit
B"H add test URLs info in case it changes