Automatically retry failed downloads 3 times

Enable the Acquire::Retries option by default, set to 3.
This will help with slightly unreliable networking; future
work is needed for adding backoff and SRV/IP rotation.

LP: #1876035
Gbp-Dch: full

apt-pkg/acquire-item.cc

@@ -770,7 +770,7 @@ class APT_HIDDEN CleanupItem : public pkgAcqTransactionItem		/*{{{*/
 // Acquire::Item::Item - Constructor					/*{{{*/
 pkgAcquire::Item::Item(pkgAcquire * const owner) :
    FileSize(0), PartialSize(0), ID(0), Complete(false), Local(false),
-    QueueCounter(0), ExpectedAdditionalItems(0), Retries(_config->FindI("Acquire::Retries", 0)), Owner(owner), d(new Private())
+    QueueCounter(0), ExpectedAdditionalItems(0), Retries(_config->FindI("Acquire::Retries", 3)), Owner(owner), d(new Private())
 {
    Owner->Add(this);
    Status = StatIdle;

test/integration/test-proxy-connect

@@ -19,4 +19,13 @@ echo "Acquire::http::Proxy \"http://localhost:${APTHTTPPORT}\";" > rootdir/etc/a
 
 aptget update >/dev/null 2>&1
 testsuccessequal "CONNECT example.example:443 HTTP/1.1\r
+Host: example.example:443\r
+--
+CONNECT example.example:443 HTTP/1.1\r
+Host: example.example:443\r
+--
+CONNECT example.example:443 HTTP/1.1\r
+Host: example.example:443\r
+--
+CONNECT example.example:443 HTTP/1.1\r
 Host: example.example:443\r" grep -A1 "^CONNECT" aptarchive/webserver.log