Correct use of variable name `errno`

Modify the names of several local variables from `errno` to `wsaerrno`
to prevent the clash of that name and the macro used to implement
`errno` found in the standard library.

Normal namespacing rules would mean that the local `errno` would hide
the global value, however the use of a `define` macro creates a name
conflict.
Ignore the default library 'LIBC.lib'

Add the flag to QW/client/qwcl.vcxproj, QW/client/qwsv.vcxproj, and
WinQuake/WinQuake.vcxproj to ignore the default library 'LIBC.lib'.

The library cannot be found by Visual Studio, and is not actually
required to build any of the projects.