ftp: add 'list_only' to the transfer state struct

and rename it from 'ftp_list_only' since it is also used for SSH and
POP3. The state is updated internally for 'type=D' FTP URLs.

Added test case 1570 to verify.

Closes #6578

tests/data/test1570

@@ -0,0 +1,73 @@
+<testcase>
+<info>
+<keywords>
+FTP
+PASV
+RETR
+</keywords>
+</info>
+# Server-side
+<reply>
+<data nocheck="yes">
+data
+    to
+      see
+that FTP
+works
+  so does it?
+</data>
+<servercmd>
+REPLY EPSV 500 no such command
+</servercmd>
+</reply>
+
+# Client-side
+<client>
+<server>
+ftp
+</server>
+ <name>
+FTP first type=D then regular URL
+ </name>
+<tool>
+lib1569
+</tool>
+<command>
+"ftp://%HOSTIP:%FTPPORT/1570;type=D" ftp://%HOSTIP:%FTPPORT/1570
+</command>
+
+</client>
+
+# Verify data after the test has been "shot"
+<verify>
+<protocol>
+USER anonymous
+PASS ftp@example.com
+PWD
+EPSV
+PASV
+TYPE A
+NLST
+PASV
+TYPE I
+SIZE 1570
+RETR 1570
+QUIT
+</protocol>
+</verify>
+<stdout>
+data
+    to
+      see
+that FTP
+works
+  so does it?
+data
+    to
+      see
+that FTP
+works
+  so does it?
+</stdout>
+
+</testcase>