miniupnp support in Windows builds

[teteros at teknik.io]

Currently Tinc Pre-release Windows binaries do not come with UPnP support compiled in.

I'm not sure what cross-build environment tinc project currently uses to make tinc 1.1 Windows binaries but in MSYS2 it's enough to depends() on miniupnpc library and add --enable-miniupnpc to configure which seems trivial.

I've been recompiling tinc with this one change for my project and UPnP seems to work very well so far on Windows nodes.

Another small minor issue with the Windows package is with the bundled tap-windows install scripts.
The batch files do not refer to the directory the scripts are in (e.g via using "%~dp0tapinstall.exe")
which means that when the batch files are ran as administrator (required for driver installs)
cmd.exe attempts to run the command from System32 rather than directory where tapinstall.exe and OemWin2k.inf files are.