gui: Don't sleep before executing the status command.

This results in uncacceptable delays when the status command fails
for any reason, for example because the server is down.

Replace this code by introducing a struct timeval variable "next_exec",
which stores the earliest possible time to restart the status command.

Move all status fetching code before the select() call in do_select()
to open_stat_pipe() and rename the function to status_pre_select(),
which is more appropriate.