log: add an "error" parameter to all low-level logging calls and intrdouce log_error_errno() as log calls that take error numbers

This change has two benefits:

- The format string %m will now resolve to the specified error (or to
  errno if the specified error is 0. This allows getting rid of a ton of
  strerror() invocations, a function that is not thread-safe.

- The specified error can be passed to the journal in the ERRNO= field.

Now of course, we just need somebody to convert all cases of this:

        log_error("Something happened: %s", strerror(-r));

into thus:

        log_error_errno(-r, "Something happened: %m");

src/test/test-hostname.c

@@ -32,7 +32,7 @@ int main(int argc, char* argv[]) {
 
         r = hostname_setup();
         if (r < 0)
-                fprintf(stderr, "hostname: %s\n", strerror(-r));
+                log_error_errno(-r, "hostname: %m");
 
         return 0;
 }