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/libutil/logging.cc

@@ -116,7 +116,7 @@ void writeToStderr(std::string_view s)
 {
     try {
         writeFull(STDERR_FILENO, s, false);
-    } catch (SysError & e) {
+    } catch (SystemError & e) {
         /* Ignore failing writes to stderr.  We need to ignore write
            errors to ensure that cleanup code that logs to stderr runs
            to completion if the other side of stderr has been closed