Harmonize TOKENBUF_SIZE and stack buf sizes

The max identifier length depends on the max tokenbuf size,
which went from 256 in perl5 to 1024 in cperl, mostly to speed up parsing
with a larger parser buffer.
Harmonize a few places which still kept a hardcoded 256.
Fixes id:000099,sig:11,src:027197,op:havoc,rep:4 from #293

Use a larger IO buffer of 1024 for pp_backtick, when draining it for the
sideeffect only.

Get rid of unnecessary sv_gets COW logic. SvGROW already does add +1.

doio.c

@@ -670,7 +670,7 @@ S_openn_cleanup(pTHX_ GV *gv, IO *io, PerlIO *fp, char *mode, const char *oname,
 	    && IoTYPE(io) != IoTYPE_WRONLY  /* Dups of STD* filehandles already have */
 	    && IoTYPE(io) != IoTYPE_RDONLY  /* type so they aren't marked as sockets */
 	) {				    /* on OS's that return 0 on fstat()ed pipe */
-	     char tmpbuf[256];
+             char tmpbuf[256];              /* SOCK_MAXADDRLEN 255 */
 	     Sock_size_t buflen = sizeof tmpbuf;
 	     if (PerlSock_getsockname(fd, (struct sockaddr *)tmpbuf, &buflen) >= 0
 		      || errno != ENOTSOCK)

mg.c

@@ -1303,7 +1303,7 @@ Perl_magic_setenv(pTHX_ SV *sv, MAGIC *mg)
             /* set MGf_TAINTEDDIR if any component of the new path is
              * relative or world-writeable */
 	    while (s < strend) {
-		char tmpbuf[256];
+		char tmpbuf[TOKENBUF_SIZE];
 		Stat_t st;
 		I32 i;
 #ifdef __VMS  /* Hmm.  How do we get $Config{path_sep} from C? */

op.c

@@ -12967,12 +12967,12 @@ Perl_ck_sort(pTHX_ OP *o)
 	    o->op_flags |= OPf_SPECIAL;
 	}
 	else if (IS_CONST_OP(kid) && kid->op_private & OPpCONST_BARE) {
-	    char tmpbuf[256];
+	    char tmpbuf[TOKENBUF_SIZE];
 	    STRLEN len;
 	    PADOFFSET off;
 	    const char * const name = SvPV(kSVOP_sv, len);
 	    *tmpbuf = '&';
-	    assert (len < 256);
+	    assert (len < TOKENBUF_SIZE);
 	    Copy(name, tmpbuf+1, len, char);
 	    off = pad_findmy_pvn(tmpbuf, len+1, SvUTF8(kSVOP_sv));
 	    if (off != NOT_IN_PAD) {

pp_sys.c

@@ -312,7 +312,7 @@ PP(pp_backtick)
 	    PerlIO_apply_layers(aTHX_ fp,mode,type);
 
 	if (gimme == G_VOID) {
-	    char tmpbuf[256];
+	    char tmpbuf[1024];
 	    while (PerlIO_read(fp, tmpbuf, sizeof tmpbuf) > 0)
 		NOOP;
 	}

sv.c

@@ -8636,13 +8636,7 @@ Perl_sv_gets(pTHX_ SV *const sv, PerlIO *const fp, STRLEN append)
 	if (fd >= 0 && (PerlLIO_fstat(fd, &st) == 0) && S_ISREG(st.st_mode))  {
 	    const Off_t offset = PerlIO_tell(fp);
 	    if (offset != (Off_t) -1 && (Off_t)(st.st_size + append) > offset) {
-#ifdef PERL_COPY_ON_WRITE
-                /* Add an extra byte for the sake of copy-on-write's
-                 * buffer reference count. */
-		(void) SvGROW(sv, (STRLEN)((st.st_size - offset) + append + 2));
-#else
 		(void) SvGROW(sv, (STRLEN)((st.st_size - offset) + append + 1));
-#endif
 	    }
 	}
 	rsptr = NULL;

toke.c

@@ -556,7 +556,7 @@ S_def_coretype(pTHX_ SV *sv, const char *t1, int len)
 
 static void
 S_no_such_class(pTHX_ char *s) {
-    char tmpbuf[1024];
+    char tmpbuf[TOKENBUF_SIZE];
     int len;
     PL_bufptr = s;
     len = my_snprintf(tmpbuf, sizeof(tmpbuf), "No such class %.1000s", PL_tokenbuf);
@@ -9468,7 +9468,7 @@ S_checkcomma(pTHX_ const char *s, const char *name, const char *what)
 		return;
 	    if (s - w <= 254) {
                 PADOFFSET off;
-		char tmpbuf[256];
+		char tmpbuf[TOKENBUF_SIZE];
 		Copy(w, tmpbuf+1, s - w, char);
 		*tmpbuf = '&';
 		off = pad_findmy_pvn(tmpbuf, s-w+1, 0);
@@ -13209,7 +13209,7 @@ Perl_parse_subsignature(pTHX)
             if (!typestash)
                 typestash = find_in_coretypes(PL_tokenbuf, len);
             if (!typestash) {
-                char tmpbuf[1024];
+                char tmpbuf[TOKENBUF_SIZE];
                 int len;
                 PL_bufptr = s;
                 len = my_snprintf(tmpbuf, sizeof(tmpbuf), "No such class %.1000s", PL_tokenbuf);