BUG/MINOR: config: Fix setting of alt_proto

BUG/MINOR: config: Fix setting of alt_proto

This patch fixes the bug presented in issue #3254
(https://github.com/haproxy/haproxy/issues/3254), which
occured on FreeBSD when using a stream socket for in
nameserver section. This bug occured due to an incorrect
reset of the alt_proto for a stream socket when the default
socket is created as a datagram socket. This patch fixes
this bug by doing a late assignment to alt_proto when
a datagram socket is requested, leaving only the modification
of alt_proto done by mptcp. Additional documentation
for the use of alt_proto has also been added to
clarify the use of the alt_proto variable.