support spaces in command-line args

On modern Windows systems, any arg that is a path value is somewhat likely to have spaces in it.

Linux and SDL builds for Quake tend to handle these correctly, but various non-SDL Windows builds don't really. Let's fix it here!

The main change is in WinMain in sys_win.c, to make sure we interpret the argument tokens from the command line correctly. See the code comments for more details.

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COM_InitArgv in common.c needs an ancillary change, to properly show arguments-with-spaces in the constructed "cmdline" console variable. From a user's POV this is muuuch less interesting than the main change, since the cmdline variable is just an informative thing (and rarely used I think). But getting it right is somewhat tricky.

The problem looks like this:

- We want to populate a var with the command line used to launch Quake.

- We are trying to reconstruct that command line from the argument vector.

- Proper reconstruction depends on the arg-quoting conventions in the environment used to launch Quake.


I settled on an approach that is not super-crossplatform-elegant but which has minimal churn to the existing code:

- COM_InitArgv will use a bash-like quoting convention for command-line reconstruction. Pretty simple stuff: quote arguments that contain whitespace, backslash-quote any actual quote chars, and backslash-quote backslash chars as needed.

- If the caller doesn't want that convention (which, in Windows, we don't) then the caller can provide the actual command line if that's available, to be used instead of reconstructing from the arg vector. On Windows, the command line is in fact available, so provide it.


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I've been using this with the Windows build... works fine, populates the cmdline var as expected. Example screenshot here from a launch using a basedir path that contains spaces:

![e1m3_000](https://cloud.githubusercontent.com/assets/2068148/2811848/a0710d3a-ce34-11e3-9b43-1e16dc75d8d4.jpg)

The Linux build compiles and starts, but I don't have a graphical Linux environment yet that can run reQuiem (it fails with XF86DGANoDirectVideoMode). So you'll probably want to sanity-test this on Linux still. Or wait until I can. :-) Maybe there's a way to get my CentOS VM (VMware Fusion) capable of running this.