drivers/modem/modem_socket: Updated API

The internal socket context struct modem_socket_config
currently has members accessed directly by user. The
modem_socket_init() function has been updated to take all
user configurations as args. Thus removing the need for
the user to directly access the internal context for
initialization.

The user also currently needs to know of internal modem
socket libary behavior to determine if a socket has been
allocated and assigned an id, this is documented, and is
not safe. The functions:
 modem_socket_is_allocated()
 modem_socket_id_is_assigned()
 modem_socket_id_assign()
have been added to the modem socket library API to perform
these checks, and to assign socket ids.

This commit makes use of the modem socket library safer and
adds documentation to the API.

Signed-off-by: Bjarki Arge Andreasen <baa@trackunit.com>