global: remove unnecessary horizontal tabs.

This change was made by applying emacs' untabify function to
nearly all files in Bison's repository.  Required tabs in make
files, ChangeLog, regexps, and test code were manually skipped.
Other notable exceptions and changes are listed below.
* bootstrap: Skip because we sync this with gnulib.
* data/m4sugar/foreach.m4
* data/m4sugar/m4sugar.m4: Skip because we sync these with
Autoconf.
* djgpp: Skip because I don't know how to test djgpp properly, and
this code appears to be unmaintained anyway.
* README-hacking (Hacking): Specify that tabs should be avoided
where not required.

tests/regression.at

@@ -532,7 +532,7 @@ AT_SETUP([Web2c Report])
 AT_KEYWORDS([report])
 
 AT_DATA([input.y],
-[[%token	undef_id_tok const_id_tok
+[[%token        undef_id_tok const_id_tok
 
 %start CONST_DEC_PART
 
@@ -542,12 +542,12 @@ CONST_DEC_PART:
         ;
 
 CONST_DEC_LIST:
-	  CONST_DEC
+          CONST_DEC
         | CONST_DEC_LIST CONST_DEC
         ;
 
 CONST_DEC:
-	  { } undef_id_tok '=' const_id_tok ';'
+          { } undef_id_tok '=' const_id_tok ';'
         ;
 %%
 ]])
@@ -1062,7 +1062,7 @@ static int yylex (void);
 start:
   {
     printf ("Bison would once convert this action to a midrule because of the"
-	    " subsequent braced code.\n");
+            " subsequent braced code.\n");
   }
   ;