maint: clarify GPLv2.1+ -> GPLv2+ in the license notices

This revision, on behalf of prevailing inclination expressed in the open
so far (see below), clarifies previously dubious license notices
impacting core source files.

Original issue that GPLv2.1 does not resolve to any recognized license
was raised at developers ML [1]. Its author also made, in parallel,
a best effort to contact contributors to affected files, so that any
concerns could be heard.

In response, one especially profound, retrospective analysis ensued [2].
Presented historical context (genesis in the heartbeat project [3],
spreading from here through copy-paste epidemy into the wider ecosystem
inadverently, to "stay in project") shed solid light into what's the
most plausible interpretation of "GNU General Public License [...],
either version 2.1 of the License, or (at your option) any later
version".

Supported also by the fact there's no controversy on the subject so far,
this is a mere practical fulfillment of pre-agreed plan[4] to let these
two unlucky, extraneous characters in the license notices rest in peace.

[1] http://oss.clusterlabs.org/pipermail/developers/2016-March/000172.html
(alt. http://article.gmane.org/gmane.comp.clustering.clusterlabs.devel/133)

[2] http://oss.clusterlabs.org/pipermail/developers/2016-March/000180.html
(alt. http://article.gmane.org/gmane.comp.clustering.clusterlabs.devel/141)

[3] http://hg.linux-ha.org/heartbeat-STABLE_3_0/rev/bc508513c4dc#l2.10

[4] http://oss.clusterlabs.org/pipermail/developers/2016-March/000183.html
(alt. http://article.gmane.org/gmane.comp.clustering.clusterlabs.devel/145)