Closes #193

Allows constructing the request first and set the URL later.  This is
needed because curl_easy_escape() is needed to construct the URL,
which however needs the CURL "easy" handle created by the Request class.

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

If libupnp is installed in a non-standard location we must rely on the
include path provided by $(pkg-config --cflags libupnp). Relative to the
path given from that command no prefix must be used to find the respective
files.

libupnp 1.6.24 added a few badly designed macros which break the MPD
build:

 https://sourceforge.net/p/pupnp/code/ci/8177a4195a4d4a5d6cb095eaf4ca5b65ac446381/

To work around this, we disable our emulation functions (from
714011c8) on this libupnp version.

Closes #163

Closes #155

Avoid another potential deadlock: if no file descriptors are
registered, our non-blocking ALSA code cannot ever work.

Bugs found by Coverity.