Statgrab compiler errors fixed

- xsysguard was compilable against libstatgrab6 (0.17) but not agains current libstatgrab9 (0.90).
- NULL passed as "entries" parameter to multiple functions.
- 1 passed to sg_init() to ignore init errors.

"Please note that the API in 0.90 has changed, so applications using libstatgrab will need to be updated to support the changes."
Statgrab user_stats API changed

- get_user_stats_name_list() and get_user_stats_num_entries() removed.
- get_user_stats_login_name(), get_user_stats_device() and get_user_stats_hostname() added.
- Other entries in sg_user_stats structure ignored for now.
Statgrab compiler warnings solved

- int fs_entries updated to size_t.
- sg_set_error() call removed, the function is not defined.