libstatgrab: fix sg_init() invocation for libstatgrab >= 0.9

In those versions, `sg_init()` need to be invoked with an additional
argument. The need for such an argument is detected at configure-time.

Fixes: #445
libstatgrab: fix sg_get_user_stats() invocation for libstatgrab >= 0.9

In those versions, `sg_get_user_stats()` need to be invoked with an
additional argument. The need for such an argument is detected at
configure-time.

Fixes: #445
libstatgrab: fix sg_get_disk_io_stats() invocation for libstatgrab >= 0.9

In those versions, `sg_get_disk_io_stats()` need to be invoked a pointer
to size_t instead of pointer to int. Such a requirement is detected at
configure-time.

Fixes: #445
Merge pull request #795 from vincentbernat/fix/libstatgrab

libstatgrab: fix sg_init() invocation for libstatgrab >= 0.9