gui: Execute stat command in status_post_select().

pre_select methods should only deal with fd sets and select timeouts,
so the part of status_pre_select() which restarts the status command
belongs to status_post_select().

Unfortunately, after this change both status_pre_select() and
status_post_select() need to know when to restart the stat command.
Therefore we had to add a new global variable next_exec.

The patch also moves status_pre_select() up so that the two functions
are close to each other.