FTP: handle "rubbish" in front of directory name in 257 responses

When doing PWD, there's a 257 response which apparently some servers
prefix with a comment before the path instead of after it as is
otherwise the norm.

Failing to parse this, several otherwise legitimate use cases break.

Bug: http://curl.haxx.se/mail/lib-2013-04/0113.html
test1217: verify parsing 257 responses with "rubbish" before path

Test 1217 verifies commit e0fb2d86c9f78, and without that change this
test fails.