* lily/output-def-scheme.cc (LY_DEFINE): take default argument.

* lily/output-def-scheme.cc (LY_DEFINE): take default  argument.

* lily/output-def.cc (lookup_variable): return SCM_UNDEFINED if undefined.

* Documentation/user/global.itely (Page formatting): document
horizontalshift.

* scm/page-layout.scm (default-page-music-height):
horizontalshift: new variable, shift all systems by
horizontalshift to the right, to make space for instrument names.

* scm/paper.scm (set-paper-dimension-variables): add horizontalshift

* lily/input-scheme.cc (LY_DEFINE): take format commands.