Add pledge(2) support for OpenBSD

pledges for i3:
"stdio rpath unix" for talking to the i3 socket usually in /tmp
"proc exec" for executing programs
"wpath cpath" are needed for the restart-in-place functionality

To make this work, @semarie pointed out that it is sufficient to ensure
that we get physical_mem_bytes only once, namely in init_logging().

pledges for i3-msg:
"stdio rpath unix" are needed for talking to the i3-socket

pledges for i3-nagbar
"rpath getpw" to find the home directory
"wpath cpath" to write the script
"proc exec" to execute the script

i3-nagbar/main.c

@@ -470,6 +470,11 @@ int main(int argc, char *argv[]) {
     font = load_font(pattern, true);
     set_font(&font);
 
+#if defined(__OpenBSD__)
+    if (pledge("stdio rpath wpath cpath getpw proc exec", NULL) == -1)
+        err(EXIT_FAILURE, "pledge");
+#endif
+
     xcb_rectangle_t win_pos = get_window_position();
 
     xcb_cursor_t cursor;