Install thermald in ${sbindir}, and point systemd service to it

Substituting @sbindir@ in thermald.service.in means that the systemd
unit will work if compiled with a non-default --sbindir, or a
non-/usr prefix.

Distributions that don't distinguish between /usr/sbin and /usr/bin
can configure with --sbindir=/usr/bin or similar.
thermald.service.in: do not depend on syslog.target

systemd uses socket activation for syslog.target, so it's unnecessary
to depend on it.
thermald.service.in: do not suppress stderr

Unless thermald's stderr is really spammy, it's better to capture
all of its output so that any error/warning messages appear in
the Journal.
Merge pull request #20 from smcv/systemd

improve systemd support; install to /sbin