Implement support for relative and absolute skeleton file names.

Implement support for relative and absolute skeleton file names.
Discussed starting at
<http://lists.gnu.org/archive/html/bison-patches/2006-12/msg00071.html>.
* doc/bison.texinfo (Decl Summary): Document in %skeleton entry.
(Bison Options): Document in --skeleton entry.
* src/output.c (output_skeleton): Use strncpy rather than strcpy since
full_skeleton can't necessarily hold all of pkgdatadir.
If the specified skeleton file name contains a `/', don't prepend
pkgdatadir.
* src/parse-gram.y (prologue_declaration): If the specified skeleton
file name contains a `/', prepend the grammar file directory.
* tests/Makefile.am (TESTSUITE_AT): Add skeletons.at.
* skeletons.at: New file.
(relative skeleton file names): New test case.
(installed skeleton file names): New test case.
* tests/testsuite.at: Include skeletons.at.

* bootstrap: Update copyright to 2007.