Translate sockets extension to Italian (missing/new pages)

[vpintidev]

Translates the previously untranslated pages of the sockets extension into Italian.

Section files:

• book.xml, setup.xml, constants.xml (complete), examples.xml, errors.xml, reference.xml
• socket.xml (Socket class), addressinfo.xml (AddressInfo class)
• changelog entities (socket-param, address-param) in language-snippets.ent

Functions:

• socket_set_block, socket_atmark, socket_cmsg_space
• socket_addrinfo_bind / connect / explain / lookup
• socket_export_stream, socket_import_stream
• socket_wsaprotocol_info_export / import / release

All EN-Revision headers are set to the current English revision, maintainer vpinti.

Note: this covers the pages that had no Italian translation yet. The older
sockets pages currently marked EN-Revision: n/a (originally by darvina, last
updated in 2005) are out of date and will be brought up to date in a separate
follow-up PR.

The branch history is intentionally granular — feel free to squash on merge.