Fix some .gitignore and .cvsignore problems.

* bootstrap (insert_sorted_if_absent): Replace all uses with...
(insert_vc_ignore): ... this new function, which prepends `/' to all
.gitignore entries before passing them to insert_sorted_if_absent.
* bootstrap.conf (vc_ignore): Set to '.cvsignore .gitignore' so that
.cvsignore files are maintained even though Bison developers run
bootstrap while using Git.
* .cvsignore (*.patch *.log log patches applied): Remove, apparently
unneeded by Bison.
(gnulib): Add.
* .gitignore (/*.patch *.log log patches applied): Remove, broken and
unneeded.  Reported by Eric Blake.
* lib/.gitignore (/*~): Add.
* po/.cvsignore, runtime-po/.cvsignore: Sync with .gitignore.
* examples/calc++/.gitignore (/calc++.exe): Add.  Reported by Eric
Blake.
* src/.gitignore (/bison.exe): Add.  Reported by Eric Blake.

bootstrap.conf

@@ -65,3 +65,7 @@ excluded_files='
     m4/visibility.m4
     m4/xsize.m4
 '
+
+# In directories like lib/, m4/, and po/, generate both these files because we
+# still try to support CVS emulation.
+vc_ignore='.cvsignore .gitignore'