bpo-38282: Correctly manage the Bluetooth L2CAP socket structure in FreeBSD

[pablogsal]

Example description:

https://man.cx/ng_btsocket(4)

https://bugs.python.org/issue38282

[pablogsal]

Tested on the FreeBSD CURRENT buildbots:

BEFORE:

Python build finished successfully!
The necessary bits to build these optional modules were not found:
_gdbm                 _tkinter              spwd               
To find the necessary bits, look in setup.py in detect_modules() for the module's name.


The following modules found by detect_modules() in setup.py, have been
built by the Makefile instead, as configured by the Setup files:
_abc                  atexit                pwd                
time                                                           


Failed to build these modules:
_socket                                                        

AFTER

Python build finished successfully!
The necessary bits to build these optional modules were not found:
_gdbm                 _tkinter              spwd               
To find the necessary bits, look in setup.py in detect_modules() for the module's name.


The following modules found by detect_modules() in setup.py, have been
built by the Makefile instead, as configured by the Setup files:
_abc                  atexit                pwd                
time                                 

[pablogsal]

I am merging this to fix the FreeBSD buildbots, please @vstinner, review this PR and build upon or add something extra if you see something incorrect/improvable. :)