Incorporate Ermal's comments -- use conf_mount_rw() only where needed.

This update incorporates Ermal's comments from Pull Request 571 (Snort pkg v3.0.2).  Unnecessary calls to conf_mount_rw() and conf_mount_ro() are removed.  A call to mount the file system read/write in the snort_generate_conf() function was found to be missing, and the call was added there.  That missing function call was the root of the original error reported on the Support Forum about "...writing to read-only file system...".

No change in core functionality is contained in this code edit, thus the Snort package version remains at 2.9.5.5 v3.0.2.