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.