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/apps/reader/modules/readerfooter.lua

@@ -177,7 +177,7 @@ local footerTextGeneratorMap = {
     time = function(footer)
         local symbol_type = footer.settings.item_prefix or "icons"
         local prefix = symbol_prefix[symbol_type].time
-        local clock = util.secondsToHour(os.time(), footer.settings.time_format == "12")
+        local clock = util.secondsToHour(os.time(), G_reader_settings:isTrue("twelve_hour_clock"))
         if not prefix then
             return clock
         else
@@ -1287,43 +1287,6 @@ function ReaderFooter:addToMainMenu(menu_items)
                     },
                 },
             },
-            {
-                text = _("Time format"),
-                sub_item_table = {
-                    {
-                        text_func = function()
-                            local footer = {}
-                            footer.settings = {}
-                            footer.settings.time_format = "24"
-                            footer.settings.item_prefix = self.settings.item_prefix or "icons"
-                            return T(_("24-hour (%1)"),footerTextGeneratorMap.time(footer))
-                        end,
-                        checked_func = function()
-                            return self.settings.time_format == "24" or self.settings.time_format == nil
-                        end,
-                        callback = function()
-                            self.settings.time_format = "24"
-                            self:refreshFooter(true)
-                        end,
-                    },
-                    {
-                        text_func = function()
-                            local footer = {}
-                            footer.settings = {}
-                            footer.settings.time_format = "12"
-                            footer.settings.item_prefix = self.settings.item_prefix or "icons"
-                            return T(_("12-hour (%1)"),footerTextGeneratorMap.time(footer))
-                        end,
-                        checked_func = function()
-                            return self.settings.time_format == "12"
-                        end,
-                        callback = function()
-                            self.settings.time_format = "12"
-                            self:refreshFooter(true)
-                        end,
-                    },
-                }
-            },
             {
                 text = _("Duration format"),
                 sub_item_table = {