remove global g_UserLibDirBuffer which was a project specific global, and no longer used with FP_LIB_TABLE support.

.kicad_mod files: always save the "last edited date" because only the footprint editor changes this parameter. Usefull to know if/when a footprint was actually modified.
Also fix a minor issue in footprint editor which always set the "modified" flag even when no change was made.

Pcbnew: fix a minor bug: in zones with no net, only the zone clerance was used to created clearence,  regardless the pad or footprint local clerance.
Very minor other fixes.

This workaround was already used (bzr 4301, in file plot_board_layers.cpp) also to avoid similar crashes (uses 18 segments instead of 16 to approximate a circle by segments).
Minor code cleanup in dialog_plot.

Fix kicad_plugin.cpp coding style fully broken by commit 4887 (patch from Orson, AKA maciej suminski).