Move the code to add magic to $0 into Perl_gv_fetchpvn_flags().

Move the code to add magic to $0 into Perl_gv_fetchpvn_flags().

Curiously, this appears to always have been implemented slightly inefficiently.
It dates from perl 4.000, which added the functionality to make assigning to $0
set the process name. The equivalent fix for 4.000 is:

--- a/perl.c
+++ b/perl.c
@@ -738,7 +738,7 @@ FIX YOUR KERNEL, PUT A C WRAPPER AROUND THIS SCRIPT, OR USE -u AND UNDUMP!\n");
 	(void)hadd(sigstab);
     }

-    magicalize("!#?^~=-%.+&*()<>,\\/[|`':\004\t\020\024\027\006");
+    magicalize("!#?^~=-%.+&*()<>,\\/[|`':0\004\t\020\024\027\006");
     userinit();		/* in case linked C routines want magical variables */

     amperstab = stabent("&",allstabs);
@@ -828,7 +828,6 @@ FIX YOUR KERNEL, PUT A C WRAPPER AROUND THIS SCRIPT, OR USE -u AND UNDUMP!\n");
 #endif
     if (tmpstab = stabent("0",allstabs)) {
 	str_set(stab_val(tmpstab),origfilename);
-	magicname("0", Nullch, 0);
     }
     if (tmpstab = stabent("\030",allstabs))
 	str_set(stab_val(tmpstab),origargv[0]);