wpa: ctrl_iface set sendbuf size

In order to avoid running into the default net.local.dgram.maxdgram
of 2K currently when calling sendto(2) try to set the sndbuf size to
the maximum ctrl message size.
While on 14 and 15 this does not actually raise the limit anymore (and
be7c095ac99ad29fd72b780c7d58949a38656c66 raised it for syslogd and this),
FreeBSD 13 still requires this change and it will work as expected there.
In addition we always ensure a large enough send buffer this way
independent of kernel defaults.
The problem occured, e.g., when the scan_list result had enough BSSIDs
so the text output would exceed 2048 bytes.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
PR: 274990
Reviewed by: cy, adrian (with previous comment)
Differential Revision: https://reviews.freebsd.org/D42558