socket(2): refactor the manual page

[glebius]

Summary

Create a chapter on every important socket type: stream, datagram,
seqpacket. Always list what protocol families do support what kinds of
sockets. Improve some statements possessing language from the
specification [1]. Reduce some statements that are mostly specific to
TCP. Provide more external links and references to various important
syscalls that can be used on sockets.

The big factual change is documentation of SOCK_SEQPACKET. In FreeBSD 15
this socket now fully follows the specification and is a stream socket
with record boundaries.

[1] https://pubs.opengroup.org/onlinepubs/9799919799/functions/V2_chap02.html#tag_16_10_06

[roy_marples.name]

lib/libsys/socket.2

246–253

I think it's worth noting that implicit boundaries are not set using write or writev anymore and the sender should use the send(2) family of functions with the MSG_EOR flag instead.

[glebius]

• Added sentence on how to set record boundaries with SOCK_SEQPACKET.

[glebius]

• Add a paragraph on non-blocking mode