Follow-up to 3463408.
Prior to 3463408 file:// hostnames were silently stripped.
Prior to this commit it did not work when a schemeless url was used with
file as the default protocol.
Ref: https://curl.haxx.se/mail/lib-2016-11/0081.html
Closes https://github.com/curl/curl/pull/1124
Also fix for drive letters:
- Support --proto-default file c:/foo/bar.txt
- Support file://c:/foo/bar.txt
- Fail when a file:// drive letter is detected and not MSDOS/Windows.
Bug: https://github.com/curl/curl/issues/1187
Reported-by: Anatol Belski
Assisted-by: Anatol Belski
