vsl sequence and segment updates didn't happen atomically, so
vslc_vsm_check could report spurious overruns.

Replace sequence and segment index with a counter (segment_n), which
always increments (with overflow after UINT_MAX). The actual segment
index will be segment_n % VSL_SEGMENTS. Overrun detection by
calculation of the difference between two segment numbers becomes
simple and safe because we only ever access/update a single integer.

Update the shared memory log head.

(struct VSLC_ptr).priv is now the the equivalent of segment_n from the
reader side. It gets initialized once and is maintained independently.

Patch prepared in collaboration with Martin Blix Grydeland
<martin@varnish-software.com>

Fixes: #1747