net: sockets: Add freeaddrinfo()

Add freeaddrinfo() to complement getaddrinfo().

Existing applications using getaddrinfo() will usually free
allocated memory using freeaddrinfo(). Even if nothing is allocated
the function should exist to avoid having to change the application
when porting.

Signed-off-by: Stig Bjørlykke <stig.bjorlykke@nordicsemi.no>