Closes https://github.com/MusicPlayerDaemon/MPD/issues/456

Similar to 5a5229b4: use more C++14
constexpr.

"De-facto" field mappings are available at http://wiki.hydrogenaud.io/index.php?title=Tag_Mapping.

Closes #343

fails on it. This fixes.

libwrap is an obscure artefact from a past long ago, when source IP
address meant something.

And its API is "interesting"; it requires the application to expose
two global variables `allow_severity` and `deny_severity`.  This led
to bug #437.  I don't want to declare those variables; instead, I'd
like to remove libwrap support.

Closes #437

Since commit cc64c715, UpdateService::Enqueue() throws
ProtocolError(ACK_ERROR_UPDATE_ALREADY) instead of returning 0.

This is similar to https://github.com/MusicPlayerDaemon/MPD/commit/b177bffa6a5f1b69639b6cd759dcb16fdd76e5e2, in that it fixes the Windows issue of connecting to the open socket. Also, the listen_socket is set to AcceptNonBlock after the connection anyways.

This function is sparsely documented and a look at the bluez-alsa
source code shows that implementations make undocumented assumptions
on the `struct pollfd` array parameter which can lead to strange
effects.

This function implements the "group" feature on top of the
Database::Visit() method.

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.