Changes

fetchers: drop support for ancient OpenSSL
We now require 1.1.1 or later (and, preferably, 3.x).
Also take the opportunity to add support for LibreSSL 3.5.0 or later (we
still support ancient versions of this because 3.5.0 is still relatively
modern -- give it a few more years and support for old LibreSSL can go,
too)

RISC OS: use modern fallthrough mechanism

RISC OS: optimisations for UTF-8<->local encoding

Fetch/curl: expose socket open/close via fetch vtable
This allows frontends to customise the behaviour of sockets. The default
implementation simply maps to socket(2)/close(2).

Socket: fix win32/aos3 build

Socket: more fixes for win32/aos3
These special snowflakes have specialised socket close APIs, so ensure
that we use the correct one. Additionally, there's no guarantee that
their socket() API signature matches the POSIX definition, so wrap it up
and cast the result.

Socket: avoid warning on aos3

RISC OS: add SocketWatch module to distribution

RISC OS: use SocketWatch for socket activity
Use SocketWatch to monitor our sockets and convert activity into a
pollword update. This allows us to ditch the schedule-based fetch path
and take the same one as we do on other platforms. This also allows us
to refactor the main poll loop such that we need only poll idle while
waiting for things to happen (once something has happened, we poll
aggressively until there are no non-pollword events left).

fix gcov flushing in assert exit