reader: clarify variable names

* src/reader.c (grammar_rule_check_and_complete): When 'p' and 'lhs'
are aliases, prefer the latter, for clarity and consistency.
(grammar_current_rule_begin): Avoid 'p', current_rule suffices.
* src/gram.h, src/gram.c: Comment changes.

ptdr#	calc.tab.c

src/parse-gram.y

@@ -666,7 +666,7 @@ rhs:
 ;
 
 named_ref.opt:
-  %empty         { $$ = 0; }
+  %empty         { $$ = NULL; }
 | BRACKETED_ID   { $$ = named_ref_new ($1, @1); }
 ;