Somewhere along the line, I misunderstood the whole FTP_PASSIVE_MODE debate

and had libfetch selecting passive mode even when FTP_PASSIVE_MODE was not
set at all, which is really quite surprising unless you know about it. So
change it to the agreed default behaviour of selecting passive mode if
FTP_PASSIVE_MODE is set, but not "no".

lib/libfetch/fetch.3

@@ -422,9 +422,9 @@ and message, e.g. "File is not available (404 Not Found)"
 .Sh ENVIRONMENT
 .Bl -tag -width FTP_PASSIVE_MODE
 .It Ev FTP_PASSIVE_MODE
-The FTP code selects passive mode by default.
-To force active mode FTP, set this variable to
-.Ql no .
+If set to anything but
+.Ql no ,
+forces the FTP code to use passive mode.
 .It Ev FTP_PASSWORD
 Default FTP password if the remote server requests one and none was
 provided in the URL.

lib/libfetch/ftp.c

@@ -464,7 +464,7 @@ _ftp_transfer(int cd, char *oper, char *file,
 
     /* passive mode */
     if (!pasv)
-	pasv = ((s = getenv("FTP_PASSIVE_MODE")) == NULL ||
+	pasv = ((s = getenv("FTP_PASSIVE_MODE")) != NULL &&
 		strncasecmp(s, "no", 2) != 0);
 
     /* find our own address, bind, and listen */