Add low-impact warning flags, fix warnings found by new flags

Add compiler flags: -Wundef, -Wfloat-equal, -Wredundant-decls, -Walloc-zero.

Fix -Wredundant-decls warnings:
- Remove stale cons_show_desktop_prefs declaration (ui.h)
- Remove duplicate connection_set_priority prototype (connection.h)
- Remove stale omemo_devicelist_configure_and_request declaration (omemo.h)

Fix potential buffer overflow in stanza.c:
- Increase pri_str and idle_str buffers from 10 to 12 bytes
  (INT_MIN = -2147483648 needs 12 bytes including NUL)

src/ui/ui.h

@@ -322,7 +322,6 @@ void cons_wrap_setting(void);
 void cons_time_setting(void);
 void cons_wintitle_setting(void);
 void cons_notify_setting(void);
-void cons_show_desktop_prefs(void);
 void cons_states_setting(void);
 void cons_outtype_setting(void);
 void cons_intype_setting(void);