LibTimeZone+Userland: Rename current_time_zone to system_time_zone

This renames the current implementation of current_time_zone to
system_time_zone to more clearly indicate what it is. Then reimplements
current_time_zone to return whatever was set up by tzset, falling back
to UTC if something went awry, for convenience.

Userland/Applications/ClockSettings/ClockSettingsWidget.cpp

@@ -20,7 +20,7 @@ ClockSettingsWidget::ClockSettingsWidget()
     load_from_gml(clock_settings_widget_gml);
 
     static auto time_zones = TimeZone::all_time_zones();
-    m_time_zone = TimeZone::current_time_zone();
+    m_time_zone = TimeZone::system_time_zone();
 
     m_time_zone_combo_box = *find_descendant_of_type_named<GUI::ComboBox>("time_zone_input");
     m_time_zone_combo_box->set_only_allow_values_from_model(true);

Userland/Libraries/LibC/time.cpp

@@ -372,7 +372,7 @@ void tzset()
     if (char* tz = getenv("TZ"); tz != nullptr)
         time_zone = tz;
     else
-        time_zone = TimeZone::current_time_zone();
+        time_zone = TimeZone::system_time_zone();
 
     auto set_default_values = []() {
         timezone = 0;

Userland/Libraries/LibTimeZone/TimeZone.cpp

@@ -7,6 +7,7 @@
 #include <AK/String.h>
 #include <LibTimeZone/TimeZone.h>
 #include <stdio.h>
+#include <time.h>
 
 namespace TimeZone {
 
@@ -68,7 +69,7 @@ private:
     FILE* m_file { nullptr };
 };
 
-StringView current_time_zone()
+StringView system_time_zone()
 {
     TimeZoneFile time_zone_file("r");
 
@@ -79,6 +80,11 @@ StringView current_time_zone()
     return "UTC"sv;
 }
 
+StringView current_time_zone()
+{
+    return canonicalize_time_zone(tzname[0]).value_or("UTC"sv);
+}
+
 ErrorOr<void> change_time_zone([[maybe_unused]] StringView time_zone)
 {
 #ifdef __serenity__

Userland/Libraries/LibTimeZone/TimeZone.h

@@ -31,6 +31,7 @@ struct NamedOffset : public Offset {
     String name;
 };
 
+StringView system_time_zone();
 StringView current_time_zone();
 ErrorOr<void> change_time_zone(StringView time_zone);
 Span<StringView const> all_time_zones();

Userland/Utilities/timezone.cpp

@@ -32,7 +32,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     }
 
     if (time_zone.is_empty()) {
-        outln("{}", TimeZone::current_time_zone());
+        outln("{}", TimeZone::system_time_zone());
         return 0;
     }