FTP: access files in root dir correctly

Accessing a file with an absolute path in the root dir but with no
directory specified was not handled correctly. This fix comes with four
new test cases that verify it.

Bug: http://curl.haxx.se/mail/lib-2013-04/0142.html
Reported by: Sam Deane

tests/data/test1225

@@ -0,0 +1,57 @@
+<testcase>
+<info>
+<keywords>
+FTP
+PASV
+RETR
+</keywords>
+</info>
+# Server-side
+<reply>
+<data>
+data
+    to
+      see
+that FTP
+works
+  so does it?
+</data>
+</reply>
+
+# Client-side
+<client>
+<server>
+ftp
+</server>
+ <name>
+FTP fetch two files using absolute paths
+ </name>
+ <command>
+ftp://%HOSTIP:%FTPPORT//foo/1225 ftp://%HOSTIP:%FTPPORT//foo/bar/1225
+</command>
+
+</client>
+
+# Verify data after the test has been "shot"
+<verify>
+<protocol>
+USER anonymous
+PASS ftp@example.com
+PWD
+CWD /
+CWD foo
+EPSV
+TYPE I
+SIZE 1225
+RETR 1225
+CWD /
+CWD /
+CWD foo
+CWD bar
+EPSV
+SIZE 1225
+RETR 1225
+QUIT
+</protocol>
+</verify>
+</testcase>