Unify the licence headers so that all same-licensed files use the exact
same text.

For some time now, libvte has been effectively LGPL3+ due to newer files being
LGPL3+ only while some older files were still nominally LGPL2+ as per their
licence headers.  Exercise the "or (at your option) any later version" upgrade
option to henceforth use, modify and distribute all these files under LGPL3+
only.