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/ui/elements/common_settings_menu_table.lua

@@ -77,10 +77,10 @@ common_settings.time = {
         text = _("12-hour clock"),
         keep_menu_open = true,
         checked_func = function()
-            return G_reader_settings:nilOrTrue("twelve_hour_clock")
+            return G_reader_settings:isTrue("twelve_hour_clock")
         end,
         callback = function()
-            G_reader_settings:flipNilOrTrue("twelve_hour_clock")
+            G_reader_settings:flipNilOrFalse("twelve_hour_clock")
         end,
         }
     }