Add get_max_sockets() and remove dead code

Add get_max_sockets() and remove dead code

The control port was using set_max_file_descriptors() with a limit set to 0
to query the number of maximum socket Tor can use. With the recent changes
to that function, a check was introduced to make sure a user can not set a
value below the amount we reserved for non socket.

This commit adds get_max_sockets() that returns the value of max_sockets so
we can stop using that "setter" function to get the current value.

Finally, the dead code is removed that is the code that checked for limit
equal to 0. From now on, set_max_file_descriptors() should never be used
with a limit set to 0 for a valid use case.

Fixes #16697

Signed-off-by: David Goulet <dgoulet@ev0ke.net>