Miscellaneous code readability improvements.

* src/reader.c (reader): Move %define front-end variable
defaults and checking into...
(prepare_percent_define_front_end_variables): ... this new
function.

* src/scan-gram.l (INITIAL): For consistency with string
literals, don't store open quote on character literal.  It's
discarded before returning anyway.
(SC_ESCAPED_CHARACTER): Similarly, don't store close quote.
Make length test more readable, and make the character stored
for an empty literal more obvious while consistent with the
previous behavior.

* src/symtab.c, src/symtab.h: Rename USER_NUMBER_ALIAS to
USER_NUMBER_HAS_STRING_ALIAS throughout.
* src/symtab.c (symbol_make_alias): Remove comment from symtab.c
that is repeated in symtab.h.  Improve argument names to make it
clear which side of the symbol-string alias pair is which.
(symbol_check_alias_consistency): Improve local variable names
for the same purpose.
* src/symtab.h (struct symbol): Make comments about aliases
clearer.
(symbol_make_alias): Improve comments and argument name.
* src/output.c (token_definitions_output): Update for rename to
USER_NUMBER_HAS_STRING_ALIAS and improve comments about aliases.
(cherry picked from commit dfaa48602d)

Conflicts:

	src/symtab.c
	src/symtab.h

src/output.c

@@ -343,10 +343,10 @@ token_definitions_output (FILE *out)
       symbol *sym = symbols[i];
       int number = sym->user_token_number;
 
-      /* At this stage, if there are literal aliases, they are part of
-	 SYMBOLS, so we should not find symbols which are the aliases
-	 here.  */
-      aver (number != USER_NUMBER_ALIAS);
+      /* At this stage, if there are literal string aliases, they are
+         part of SYMBOLS, so we should not find their aliased symbols
+         here.  */
+      aver (number != USER_NUMBER_HAS_STRING_ALIAS);
 
       /* Skip error token.  */
       if (sym == errtoken)