Match WM_CLIENT_MACHINE to gethostname()

That seems to be the correct comparison to make [1].  Studying the
hostname(1) source reveals that the default mode (running without
arguments) returns the output of gethostname(2) directly.

Fixes #3

[1] http://standards.freedesktop.org/wm-spec/1.3/ar01s07.html#KILLINGWINDOWS
0.4 Release