chg: better error check and debug stuff for newstruct/blackbox
fix: assign of derived newstructs
fix: compiler warning
Fix for Cygwin: cast to pointer from integer of different size
Merge pull request #428 from jankoboehm/spielwiese

Fix for Cygwin: cast to pointer from integer of different size
chg: simplified npNegM
chg: use imagemagick instead of xv for display

from master
chg: r_IsRingVar (extensions may not be singular rings, but uuse coeffs interface)
chg: factory/libfac is not optional, removing HAVE_FACTORY/HAVE_LIBFAC
fix: gcc optimizer bug: need local number variable
flint stuff: move optional parts to the end in coeffs
documenation of divisor.lib
chg: debug message: also for proc outside of libs