srv/s_conf: Config error message moved here.

This change references the following charybdis commits:

 - atheme@charybdis/2c0450fb601299ac27dab57aba81e501ec50550a src/s_conf: Avoid re-inventing the wheel
 - atheme@charybdis/a576a0fe64158002cd868e8a0dd72ff9ccbcfecd src/s_conf: More detailed error messages conforming to POSIX errno

shadowircd will now report the POSIX error message when unable to open the configuration file.