lib: gx_mkbasedir function should return negated errno

Callers expect it like that, and

```
E-1752: link_to /var/lib/gromox/user/domain.de/abc/eml/R-a6/4011ccdb660c46ee8b55750cc63f23a6: No such file or directory
```

should be(come)

```
E-1941: mkdir R-a6/4011ccdb660c46ee8b55750cc63f23a6: Permission denied
```

Fixes: gromox-2.48-62-g85bba8952
References: GXH-162

Author: jengelh

lib/rfbl.cpp

@@ -1843,8 +1843,7 @@ int gx_mkbasedir(const char *file, unsigned int mode)
 		mode |= S_IXGRP;
 	if (mode & (S_IROTH | S_IWOTH))
 		mode |= S_IXOTH;
-	auto ret = HX_mkdir(base.get(), mode);
-	return ret < 0 ? -ret : 0;
+	return HX_mkdir(base.get(), mode);
 }
 
 }