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/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__