Fix strange %define locations for default values.

Reported by Akim Demaille at
<http://lists.gnu.org/archive/html/bug-bison/2007-12/msg00001.html>
and discussed again starting at
<http://lists.gnu.org/archive/html/bison-patches/2008-11/msg00102.html>.
* data/bison.m4 (b4_percent_define_default): Leave syncline blank
because location information is bogus.
Use angle brackets to delimit fake file name because square brackets
look like over-quoted m4.  Choose a better fake file name.
Use negative line numbers.
* src/muscle_tab.c (muscle_percent_define_default): Likewise.
* src/location.c (location_print): If line for a boundary is negative,
only print that boundary's file name.
* src/location.h: Document that.
* tests/skeletons.at (%define Boolean variables: invalid skeleton
defaults): Update output.