Issue 2853: Set up indent-tabs-mode for Emacs in lexer.ll and parser.yy

Issue 2853: Set up indent-tabs-mode for Emacs in lexer.ll and parser.yy

indent-tabs-mode has been switched off for C++ files generally via
directory-local variables when editing with Emacs.

However, for lack of working better indentation modes that cooperate
well with Bison and Flex files, lexer.ll and parser.yy are using a
tab-based indentation style oriented on the Linux kernel indentation
style.  In order to not cause an even greater inconsistent mess, local
file variables are used for switching tab-based indentation back on in
those files when editing them with Emacs.