maint: Fix various problems with GPL copyright statements. * libgui/qterminal/libqterminal/QTerminal.cc, libgui/qterminal/libqterminal/QTerminal.h, libgui/qterminal/libqterminal/win32/QTerminalColors.cpp, libgui/qterminal/libqterminal/win32/QTerminalColors.h, libgui/qterminal/libqterminal/win32/QWinTerminalImpl.cpp, libgui/qterminal/libqterminal/win32/QWinTerminalImpl.h: Replace "Foobar" with "This program" in GPL statement. * scripts/linear-algebra/linsolve.m: Replace "This program" with "Octave" in * GPL statement. * scripts/miscellaneous/getappdata.m, scripts/miscellaneous/isappdata.m, scripts/miscellaneous/rmappdata.m, scripts/miscellaneous/setappdata.m: Add "This file is part of Octave" line. Replace "This program" with "Octave" in GPL statement. Update GPL version to 3. * scripts/plot/util/copyobj.m, scripts/plot/util/hdl2struct.m, scripts/plot/util/struct2hdl.m, scripts/polynomial/ppder.m, scripts/polynomial/ppint.m, scripts/polynomial/ppjumps.m, scripts/sparse/bicg.m, scripts/strings/untabify.m: Replace "This program" with "Octave" in GPL statement.