Separate SystemError from SysError

Most of this is a `catch SysError` -> `catch SystemError` sed. This
is a rather pure-churn change I would like to get out of the way. **The
intersting part is `src/libutil/error.hh`.**

On Unix, we will only throw the `SysError` concrete class, which has
the same constructors that `SystemError` used to have.

On Windows, we will throw `WinError` *and* `SysError`. `WinError`
(which will be created in a later PR), will use a `DWORD` instead of
`int` error value, and `GetLastError()`, which is the Windows equivalent
of the `errno` machinery. Windows will *also* use `SysError` because
Window's "libc" (MSVCRT) implements the POSIX interface, and we use it
too.

As the docs describe, while we *throw* one of the 3 choices above (2
concrete classes or the alias), we should always *catch* `SystemError`.
This ensures no matter how the implementation changes for Windows (e.g.
between `SysError` and `WinError`) the catching logic stays the same
and stays correct.

Co-Authored-By volth <volth@volth.com>
Co-Authored-By Eugene Butler <eugene@eugene4.com>

src/nix-build/nix-build.cc

@@ -148,7 +148,7 @@ static void main_nix_build(int argc, char * * argv)
                             args.push_back(word);
                 }
             }
-        } catch (SysError &) { }
+        } catch (SystemError &) { }
     }
 
     struct MyArgs : LegacyArgs, MixEvalArgs