Expect OnSocketReady() to cancel events.  If it returns false, the
SocketMonitor may be destructed already.  This fixes a use-after-free
bug in the "httpd" output plugin.

Keep the SocketMonitor registered.  This wrong return value was added
6 years ago in commit 72cf8dd8, andd
apparently, nobody ever noticed.

This missing piece probably never really hurt, because
HttpdClient::OnSocketClosed() would be called right after a socket
error, but it's better to be explicit about closing on error.

Allows removing a CommandResult::CLOSE check.

This catches lines within command lists as well.

Eliminates a temporary copy.

Fixes:

 src/net/IPv4Address.hxx: In member function 'constexpr IPv4Address::operator SocketAddress() const':
 src/net/IPv4Address.hxx:171:24: error: a reinterpret_cast is not a constant expression
   171 |   return SocketAddress((const struct sockaddr *)&address,
       |                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 src/net/IPv6Address.hxx: In member function 'constexpr IPv6Address::operator SocketAddress() const':
 src/net/IPv6Address.hxx:138:24: error: a reinterpret_cast is not a constant expression
   138 |   return SocketAddress((const struct sockaddr *)&address,
       |                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Closes https://github.com/MusicPlayerDaemon/MPD/issues/522