* etags.c [ETAGS_REGEXP]: All the new code that deals with regexps

* etags.c [ETAGS_REGEXP]: All the new code that deals with regexps
	is compiled if this is defined.  The new functions and variables
	added #ifdef ETAGS_REGEXP are not listed in this ChangeLog.
	[VMS]: All VMS specific code previously contained in
	etags-vmslib.c is now included here, modified for dealing with
	language and regex options intermixed with filenames.
	(header_file): Global variable deleted.
	(Lang_Function): New typedef.  All language parser functions
	changed to this new type.
	(string_numeric_p, substr, prestr): Functions deleted.
	(readline_internal): Does the job that readline did previously.
	(longopts): --language and --regex options added.
	(lang_names, lang_extensions, lang_func, print_language_names):
	New structures, variables and functions for choosing languages.
	(print_help): Help strings updated.  Calls print_language_names.
	(argument_type, ARGUMENT): Typedefs for dealing with language and
	regex options intermixed with filenames.
	(main): Changed the way of dealing with arguments on the command
	line to deal with language and regex options intermixed with
	filenames.
	(get_language, default_C_entries, Cplusplus_entries,
	Cstar_entries, Yacc_entries, just_read_file): New functions.
	(find_entries): Use the new method for choosing the language.
	(Pascal_functions): Allow intermixing of comment styles.
	(prolog_getit, skip_comment): Rewritten for speed.
	(readline): Rewritten to deal with regexps.