url: Fix parsing for when 'file' is the default protocol

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

tests/data/test1146

@@ -0,0 +1,45 @@
+<testcase>
+<info>
+<keywords>
+FILE
+--proto-default
+</keywords>
+</info>
+
+<reply>
+<data>
+foo
+   bar
+bar
+   foo
+moo
+</data>
+</reply>
+
+# Client-side
+<client>
+<server>
+file
+</server>
+<name>
+--proto-default file
+</name>
+<command>
+--proto-default file %PWD/log/test1146.txt
+</command>
+<file name="log/test1146.txt">
+foo
+   bar
+bar
+   foo
+moo
+</file>
+</client>
+
+# Verify data after the test has been "shot"
+<verify>
+<errorcode>
+0
+</errorcode>
+</verify>
+</testcase>