ylwrap: various fixes

ylwrap: various fixes

Rename properly header guards in generated header files, instead of
leaving Y_TAB_H.

Convert header guards in implementation files.  Because ylwrap failed
to rename properly #include in the implementation files, current
versions of Bison (e.g., 2.7) duplicate the generated header file in
the implementation file.  The header guard then protects the
implementation file from duplicate definitions from the header file.

Generate header guards with a single '_' for series of non alphabetic
characters, instead of several.  This is what Bison does.

Makes the test t/yacc-d-basic.sh pass again.

* lib/ylwrap (guard): Properly honor $1 to rename properly the
header guards.
Keep a single _ instead of several.
(rename_sed): Rename as...
(sed_fix_filenames): this.
Suggested by Stefano Lattarini.
(sed_fix_header_guards): New.
Use it.