Adjust preferred_hosts implementation

Use 0 as the reported ping time for hosts in preferred_hosts, since the
previously used value of 1 is not guaranteed to be lower than hosts
that respond extremely quickly. 0 was previously used to indicate no
entry in the cache, so switch to using the empty string for that.

Closes: #351
(cherry picked from commit ed1bda2)

src/macports1.0/macports.tcl

@@ -6273,13 +6273,13 @@ proc macports::get_pingtime {host} {
         variable preferred_hosts
         foreach pattern $preferred_hosts {
             if {[string match -nocase $pattern $host]} {
-                dict set host_cache $host 1
-                return 1
+                dict set host_cache $host 0
+                return 0
             }
         }
-        dict set host_cache $host 0
+        dict set host_cache $host {}
     }
-    if {[dict get $host_cache $host] != 0} {
+    if {[dict get $host_cache $host] ne {}} {
         return [dict get $host_cache $host]
     }
 

src/macports1.0/tests/macports.test

@@ -974,7 +974,7 @@ test get_pingtime {
     if {[macports::get_pingtime macports_blacklist] != -1} {
         return "FAIL: wrong time for blacklisted host"
     }
-    if {[macports::get_pingtime macports_pref] != 1} {
+    if {[macports::get_pingtime macports_pref] != 0} {
         return "FAIL: wrong time for preferred host"
     }
     return "Get ping time successful."