update to 1.6.0 with official syslog support backported from the 1.7 branch

tested by several, ok sthen@
Add regress test for explicit_bzero.
Add regress tests for timingsafe_bcmp and timingsafe_memcmp.

timingsafe_memcmp tests are disabled for now, pending its addition to
libc.
Add timingsafe_memcmp().

ok deraadt, jmc, tedu