Compiler messaging mega patch.

va_yyreport() is now the main C-level dispatcher for compiler messages.
It also handles the automatic conversion of type errors to warnings in compat mode.
Consolidated the location for the implementation of most of the yy*-messaging functions to program.c.
Added yytype_report().
Changed APIs for yyexplain_*().
Moved the definitions of REPORT_* from pike_compiler.h to program.h.
va_yyerror() and low_yyerror() are no more.
Most type-system messages should now be marked with the appropriate subsystem.
Reenabled the typechecking for sscanf and sprintf() now that errors are converted into warnings in compat mode.

Rev: src/language.yacc:1.425
Rev: src/las.c:1.413
Rev: src/las.h:1.79
Rev: src/modules/sprintf/sprintf.c:1.153
Rev: src/pike_compiler.h:1.14
Rev: src/pike_types.c:1.337
Rev: src/pike_types.h:1.118
Rev: src/program.c:1.702
Rev: src/program.h:1.248
Rev: src/sscanf.c:1.184