Issue 2939: Fix relations between \maininput, EOF and safe mode

Issue 2939: Fix relations between \maininput, EOF and safe mode

In lexer.ll, assumptions about the depth of include_stack_ were hardwired
that only were valid for parsers started from the normal chain of input.

One consequence was that something like

    #{ \book { \include "line-arrows.ly" } #}

worked even in safe mode (where \include should be prohibited) but
might complain about unexpected EOF.  This commit adds a member
variable main_input_level to Lily_lexer for keeping track of when to
drop restrictions and deliver EOF in each parser separately.