reader: reduce the "scope" of global variables

We have too many global variables, adding structure would help.  For a
start, let's hide some of the variables closer to their usage.

* src/getargs.c, src/files.h (current_file): Move to...
* src/scan-gram.c: here.
* src/scan-gram.h (gram_in, gram__flex_debug): Remove, make them
private to the scanner.
* src/reader.h, src/reader.c (reader): Take a grammar file as argument.
Move the handling of scanner variables to...
* src/scan-gram.l (gram_scanner_open, gram_scanner_close): here.
(gram_scanner_initialize): Remove, replaced by gram_scanner_open.
* src/main.c: Adjust.

src/getargs.c

@@ -853,7 +853,7 @@ getargs (int argc, char *argv[])
       usage (EXIT_FAILURE);
     }
 
-  current_file = grammar_file = uniqstr_new (argv[optind]);
+  grammar_file = uniqstr_new (argv[optind]);
   MUSCLE_INSERT_C_STRING ("file_name", grammar_file);
 }