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.

This compile-time check got lost during the Meson transition.

This version should have been incremented in 0.21.2 because this
version added new search operators.  In 0.21.4, we'll have more
protocol bug fixes which may be important for clients to know.