The current http output doesn't provide a header for cross-origin support. This prevents to use the mpd http stream directly from an other webapplication due the origin from the webpage differs from then the audio stream.

The fix is to add the following header to the http response:
Access-Control-Allow-Origin: *

To make things simpler, AllocatedString is now a non-template class.

Fixes Windows compatibility.

To make things simpler, AllocatedString is now a non-template class.

Similar to commits 1686f4e8 and
30a5dd26

The ones in std have overloads for const char/char.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with readability-uppercase-literal-suffix
Signed-off-by: Rosen Penev <rosenp@gmail.com>

[[maybe_unused]] (introduced in C++17) is standard C++.

https://clang.llvm.org/docs/AttributeReference.html#maybe-unused-unused
says that this is equivalent to the GNU unused attribute.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

The former is deprecated by C++14. The standard says they are the same:

The header defines all types and macros the same as the C standard library
header<stdint.h>.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

The former is deprecated with C++14. The standard says both are the same:

The contents and meaning of the header<cstddef>are the same as the C
standard library header<stddef.h>,except that it does not declare the type
wchar_t, that it also declares the type byte and its associated
operations (21.2.5), and as noted in 21.2.3 and 21.2.4.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

The former was deprecated with C++14.

According to the C++11 and C++17 standards, both files are identical.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with readability-uppercase-literal-suffix
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Return `404 not found` for some common well-known paths, as clients requesting them usually do that automatically and don't expect endless audio stram.

Closes #572

memcmp use may result in out of bounds access

Just like std::condition_variable, which however has no way to specify
the std::mutex directly.

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.