* doc/bison.texinfo: Update.

* doc/bison.texinfo: Update.
* doc/bison.1 (mandoc): Update.
* src/system.h (EXT_GUARD_C, EXT_STYPE_H): Remove .c and .h.
* src/files.c: Support output files extensions computing.
(src_extension): New static variable.
(header_extension): New static variable.
(tr): New function.
(get_extension_index): New function, gets the index of an extension
filename in a string.
(compute_exts_from_gf): New function, computes extensions from the
grammar file extension.
(compute_exts_from_src): New functions, computes extensions from the
C source file extension, file given by ``-o'' option.
(compute_base_names): Update.
(output_files): Update.