s/sigwaitinfo()/sigwait()/, etc

sigwaitinfo() isn't available on all systems we care about,
and the loss of informative killer pid/uid messages isn't
worth autoconf/ifdef compatibility mess, so downgrade to
using the more-widely available sigwait() in basically the
same fashion.
Fix Linux version detection for initial 3.x kernels

Upstream Linux's version numbering changed with 3.0: the "2.6" prefix
was replaced with just "3" and hence initial versions of kernels have
only two components, major & minor (e.g. 3.2). Subsequent stable release
updates have three parts, which is equivalent to the four parts that 2.6
stable updates had.

gdnsd_linux_min_version() assumes that three components *must* exist and
hence is broken with the initial versions of all 3.x kernels. Fix this
in a backwards- and forwards-compatible manner by assuming z=0 for
kernels that do not match x.y.z but match x.y.
Merge pull request #72 from paravoid/fix-linux-min-version

Fix Linux version detection for initial 3.x kernels
A batch of 2.x documentation updates
Merge branch 'master' into 2.x
Merge branch '2.x' into systemd