Adjust open file descriptor limit on startup

This raises qbt's open file descriptor limit to the available maximum
(within user privileges) and thus users don't need to adjust it
manually anymore.

src/app/main.cpp

@@ -33,6 +33,9 @@
 #include <cstdlib>
 #include <memory>
 
+#if defined(Q_OS_UNIX)
+#include <sys/resource.h>
+#endif
 #if !defined Q_OS_WIN && !defined Q_OS_HAIKU
 #include <unistd.h>
 #elif defined Q_OS_WIN && defined DISABLE_GUI
@@ -116,9 +119,17 @@ void displayBadArgMessage(const QString &message);
 void showSplashScreen();
 #endif  // DISABLE_GUI
 
+#if defined(Q_OS_UNIX)
+void adjustFileDescriptorLimit();
+#endif
+
 // Main
 int main(int argc, char *argv[])
 {
+#if defined(Q_OS_UNIX)
+    adjustFileDescriptorLimit();
+#endif
+
     // We must save it here because QApplication constructor may change it
     bool isOneArg = (argc == 2);
 
@@ -397,3 +408,16 @@ bool userAgreesWithLegalNotice()
 
     return false;
 }
+
+#if defined(Q_OS_UNIX)
+void adjustFileDescriptorLimit()
+{
+    rlimit limit {};
+
+    if (getrlimit(RLIMIT_NOFILE, &limit) != 0)
+        return;
+
+    limit.rlim_cur = limit.rlim_max;
+    setrlimit(RLIMIT_NOFILE, &limit);
+}
+#endif