* Makefile.am:
Building excel plugin as shared library, added plugin.xml.

* boot.[ch]:
xxx_debug variables from src/main.c moved here.
Converted to real plugin with file_opener and file_saver services.

* plugin.xml: New file.

* gnumeric/plugins/excel/excel.h, plugins/excel/ms-chart.c,
plugins/excel/ms-escher.c, plugins/excel/ms-excel-read.c,
plugins/excel/ms-excel-read.h, plugins/excel/ms-excel-util.c,
plugins/excel/ms-excel-write.c, plugins/excel/ms-excel-write.h,
plugins/excel/ms-formula-read.c, plugins/excel/ms-formula-write.c,
plugins/excel/ms-obj.c:
Adjusted function signatures, don't return TRUE/FALSE if using
IOContext (we don't need boolean value as IOContext now sets
"error_occurred" flag).