ReaderFooter: Don't duplicate a 12h clock time format option (#6973)

* ReaderFooter:
	* Honor the global twelve_hour_clock setting, instead of
	  duplicating a local one.
	  (Re #6969)

* os.date is a thin wrapper around strftime, so we might be able to get
away with some not-quite-standard extensions...

These are *definitely* supported on Linux, but are *NOT* the glibc
extension (that'd be e.g., %-I), so, hopefully, they're somewhat
portable...

They are also supported on BSD/macOS.
They are *not* supported by the MS UCRT. That means MinGW-w64, too.
This *appears* to be supported on current Bionic (it might even support
said glibc format altering extensions).

* And of course, Windows is terrible, so, make this terribly ugly to not
break it there...

* Turns out BSD also supports the dash trim format extension, so, leave
the trimming to the libc, and handle the special-casing in a way that
doesn't create stupid locals.

* Random unrelated cleanup ^^.

(https://gitter.im/koreader/koreader?at=5fd24be492aa1c4ef5d11f31)

* Update the testsuite

(Because the default used to be 24h clock).

Changed the default to 24h clock ;p.

* Explain why we don't try to fix it in Lua

frontend/util.lua

@@ -224,21 +224,40 @@ end
 ---- @int seconds number of seconds
 ---- @bool twelve_hour_clock
 ---- @treturn string hour string
-function util.secondsToHour(seconds, twelve_hour_clock)
-    local time
-    if twelve_hour_clock then
-        if os.date("%p", seconds) == "AM" then
-            -- @translators This is the time in the morning in the 12-hour clock (%I is the hour, %M the minute).
-            time = os.date(_("%I:%M AM"), seconds)
+--- @note: The MS CRT doesn't support either %l & %k, or the - format modifier (as they're not technically C99 or POSIX).
+---        They are otherwise supported on Linux, BSD & Bionic, so, just special-case Windows...
+---        We *could* arguably feed the os.date output to gsub("^0(%d)(.*)$", "%1%2"), but, while unlikely,
+---        it's conceivable that a translator would put something other that the hour at the front of the string ;).
+if jit.os == "Windows" then
+    function util.secondsToHour(seconds, twelve_hour_clock)
+        if twelve_hour_clock then
+            if os.date("%p", seconds) == "AM" then
+                -- @translators This is the time in the morning using a 12-hour clock (%I is the hour, %M the minute).
+                return os.date(_("%I:%M AM"), seconds)
+            else
+                -- @translators This is the time in the afternoon using a 12-hour clock (%I is the hour, %M the minute).
+                return os.date(_("%I:%M PM"), seconds)
+            end
         else
-            -- @translators This is the time in the afternoon in the 12-hour clock (%I is the hour, %M the minute).
-            time = os.date(_("%I:%M PM"), seconds)
+            -- @translators This is the time using a 24-hour clock (%H is the hour, %M the minute).
+            return os.date(_("%H:%M"), seconds)
+        end
+    end
+else
+    function util.secondsToHour(seconds, twelve_hour_clock)
+        if twelve_hour_clock then
+            if os.date("%p", seconds) == "AM" then
+                -- @translators This is the time in the morning using a 12-hour clock (%-I is the hour, %M the minute).
+                return os.date(_("%-I:%M AM"), seconds)
+            else
+                -- @translators This is the time in the afternoon using a 12-hour clock (%-I is the hour, %M the minute).
+                return os.date(_("%-I:%M PM"), seconds)
+            end
+        else
+            -- @translators This is the time using a 24-hour clock (%-H is the hour, %M the minute).
+            return os.date(_("%-H:%M"), seconds)
         end
-    else
-        -- @translators This is the time in the 24-hour clock (%H is the hour, %M the minute).
-        time = os.date(_("%H:%M"), seconds)
     end
-    return time
 end
 
 --- Converts timestamp to a date string