Use faster method to determine open file count

QDir is VERY slow for this calculation, because all related
QDir methods require construction of the entry list upfront,
which involves a bunch of extra work in calculating file attributes

In a simple benchtest of trying to open 1000s of files using
the ogr provider ~25% of the time was spent in calculating
open file count. Afterwards this drops to <0.5%.

src/core/qgsfileutils.cpp

@@ -30,6 +30,7 @@
 // For getrlimit()
 #include <sys/resource.h>
 #include <sys/time.h>
+#include <dirent.h>
 #endif
 
 #ifdef _MSC_VER
@@ -561,10 +562,21 @@ int QgsFileUtils::openedFileLimit()
 int QgsFileUtils::openedFileCount()
 {
 #ifdef Q_OS_LINUX
-  int res = static_cast<int>( QDir( "/proc/self/fd" ).entryList().size() );
-  if ( res == 0 )
-    res = -1;
-  return res;
+  int fileCount = 0;
+
+  DIR *dirp = opendir( "/proc/self/fd" );
+  if ( !dirp )
+    return -1;
+
+  while ( struct dirent *entry = readdir( dirp ) )
+  {
+    if ( entry->d_type == DT_REG )
+    {
+      fileCount++;
+    }
+  }
+  closedir( dirp );
+  return fileCount;
 #else
   return -1;
 #endif