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-msg/main.c

@@ -119,6 +119,10 @@ static yajl_callbacks reply_callbacks = {
 };
 
 int main(int argc, char *argv[]) {
+#if defined(__OpenBSD__)
+    if (pledge("stdio rpath unix", NULL) == -1)
+        err(EXIT_FAILURE, "pledge");
+#endif
     char *env_socket_path = getenv("I3SOCK");
     if (env_socket_path)
         socket_path = sstrdup(env_socket_path);