CONFIG REWRITE: fixed ENOENT (No such file or directory) error

how to reproduce:
1. config-file use relative path
2. option 'dir' set to another path
3. execute cmd CONFIG REWRITE
4. report "ERR Rewriting config file: No such file or directory"

the code:
```c
resetServerSaveParams();
loadServerConfig(configfile,options); /* cwd is changed here */
sdsfree(options);
/* get the wrong absolute path */
if (configfile) server.configfile = getAbsolutePath(configfile); 
```