ck_backoff: Replace ck_backoff_eb with ck_backoff_gb.

The distinction between additive/exponential implementation
and geometric implementation does little but confuse users.
The terminology used in ck_backoff now reflects terminology
used in literature.

ck_backoff_gb has been removed.
ck_ring: Add a work-around for compiler bug(s) in ck_ring_dequeue_spmc.

Both LLVM-backed compilers and GCC incorrectly treat
a barrier-sandwiched load as a loop invariant in dequeue_spmc.
Forcing volatile atomic load semantics generates the right
thing.

Thanks to Devon O'Dell and Abel Mathew for help in catching
this issue.
ck_ring: Memoize mask for SPMC operations.
ck_ring: Memoize mask across all operations.
regressions/ck_ring: Use variable for ring size.
regressions/ck_ring: Add ck_ring_spmc to fast path test.
ck_ring: Use regular assignment in type specialized ck_ring initialization.
regressions/ck_hp: Do not use empty loop body.

Fixes some GCC warnings in 4.7+.
regressions/common: Add missing header files in common.h
regressions/ck_pr: Utilize _GNU_SOURCE for ck_pr_store unit test.
ck_bytelock: Fix downgrade path from writer to reader.

An off-by-one was introduced in downgrade path from writer.
This can cause deadlock if a writer downgrades from a write lock.

Pointed out by Jeffrey Birnbaum <jmb...@...>.
Merge branch 'master' of git.concurrencykit.org:ck
legal: Update Copyright statements.
legal/doc: Update Copyright statements.
legal: Update top-level LICENSE file.
regressions/ck_pr: Place double tests under feature flag.
regressions/ck_hs: Do not rely on ck_pr_add_64 for benchmarks.

ck_hs now officially targets the 32-bit ports. No advantage
to using ck_pr_*_64 operations here.
regressions: Enable rdtsc for x86-32 targets.
Merge branch 'master' of github.com:sbahra/ck
ck_queue: Fix CK_LIST_INSERT_HEAD.

CK_LIST_INSERT_HEAD was incorrectly managing prev
pointer on insertion to non-empty list. This bug
would cause erroneous behavior on CK_LIST_REMOVE
to non-head elements. Unit test will be updated
for this regression.