Deprecate syl, add new function sylvester. * sylvester.cc: New function * syl.cc: Remove syl C++ function. * scriptss/deprecated/syl.m: m-file to replace syl.cc * NEWS: Announce new sylvester function. Announce deprecation of syl. * linalg.txi: Replace reference to syl with sylvester. * libinterp/corefcn/module.mk: Add sylvester.cc to build system. * scripts/deprecated/module.mk: Add syl.m to build system. * CMatrix.cc (Sylvester), dMatrix.cc (Sylvester), fCMatrix.cc (Sylvester), fMatrix.cc (Sylvester): Return +C rather than -C to conform to changed definition of Sylvester equation.