Add no_ctrl_interface config param to clear ctrl_interface

Add no_ctrl_interface config param to clear ctrl_interface

This can be used to override previously set ctrl_interface value in a
way that clears the variable to NULL instead of empty string. The only
real use case for this is to disable per-interface ctrl_interface from
the additional control file (-I<file>) in case ctrl_interface was set in
the main configuration file. It should be noted that zero-length
ctrl_interface parameter can be used to initiate some control interface
backends, so simpler designs were not available for this.

The format of the new parameter is not exactly cleanest due to
configuration file parsing assumptions. For example:

ctrl_interface=....
no_ctrl_interface=

would end up with ctrl_interface=NULL.

Signed-hostap: Jouni Malinen <jouni@qca.qualcomm.com>