Import rc syntax `delim{cmd} for splitting commands from 9atom.

This allows an elegant, concise way of handling parsing the
output of commands.

Reference: /n/sources/patch/rc-line-split
Date: Fri Dec  6 05:21:41 GMT 2019
Signed-off-by: ori@eigenstate.org

--- /sys/man/1/rc	Fri Dec  6 05:19:38 2019
+++ /sys/man/1/rc	Fri Dec  6 05:19:32 2019
@@ -233,6 +233,8 @@
 separated by spaces.  A variable with zero elements yields the empty string.
 .HP
 .BI `{ command }
+.HP
+.BI ` "split " { command }
 .br
 .I rc
 executes the
@@ -245,6 +247,8 @@
 .B $ifs
 is not otherwise set, its value is
 .BR "'\ \et\en'" .
+In the second form of the command, split is used instead of
+.BR $ifs .
 .HP
 .BI <{ command }
 .HP
--- /sys/src/cmd/rc/code.c	Fri Dec  6 05:19:46 2019
+++ /sys/src/cmd/rc/code.c	Fri Dec  6 05:19:41 2019
@@ -84,6 +84,8 @@
 {
 	int p, q;
 	tree *tt;
+	char *ifs;
+
 	if(t==0)
 		return;
 	if(t->type!=NOT && t->type!=';')
@@ -131,10 +133,22 @@
 		emitf(Xconc);
 		break;
 	case '`':
+ 		emitf(Xmark);
+ 		if(c0){
+ 			outcode(c0, 0);
+ 			emitf(Xglob);
+ 		} else {
+			if((ifs = strdup("ifs")) == nil)
+				sysfatal("strdup: %r");
+ 			emitf(Xmark);
+ 			emitf(Xword);
+ 			emits(ifs);
+ 			emitf(Xdol);
+ 		}
 		emitf(Xbackq);
 		if(havefork){
 			p = emiti(0);
-			outcode(c0, 0);
+			outcode(c1, 0);
 			emitf(Xexit);
 			stuffdot(p);
 		} else
--- /sys/src/cmd/rc/exec.c	Fri Dec  6 05:19:56 2019
+++ /sys/src/cmd/rc/exec.c	Fri Dec  6 05:19:50 2019
@@ -195,7 +195,7 @@
  * Xappend(file)[fd]			open file to append
  * Xassign(name, val)			assign val to name
  * Xasync{... Xexit}			make thread for {}, no wait
- * Xbackq{... Xreturn}			make thread for {}, push stdout
+ * Xbackq(split){... Xreturn}		make thread for {}, push stdout
  * Xbang				complement condition
  * Xcase(pat, value){...}		exec code on match, leave (value) on
  * 					stack
--- /sys/src/cmd/rc/havefork.c	Fri Dec  6 05:20:03 2019
+++ /sys/src/cmd/rc/havefork.c	Fri Dec  6 05:19:59 2019
@@ -84,11 +84,12 @@
 	char *stop;
 	char utf[UTFmax+1];
 	io *f, *wd;
-	var *ifs = vlook("ifs");
 	word *v, *nextv;
 	Rune r;
 
-	stop = ifs->val? ifs->val->word: "";
+ 	stop = "";
+ 	if(runq->argv && runq->argv->words)
+ 		stop = runq->argv->words->word;	
 	if(pipe(pfd)<0){
 		Xerror("can't make pipe");
 		return;
@@ -132,6 +133,7 @@
 		s_free(word);
 		closeio(f);
 		Waitfor(pid, 0);
+		poplist();	/* ditch split in "stop" */
 		/* v points to reversed arglist -- reverse it onto argv */
 		while(v){
 			nextv = v->next;
--- /sys/src/cmd/rc/pcmd.c	Fri Dec  6 05:20:09 2019
+++ /sys/src/cmd/rc/pcmd.c	Fri Dec  6 05:20:06 2019
@@ -33,7 +33,7 @@
 	break;
 	case '^':	pfmt(f, "%t^%t", c0, c1);
 	break;
-	case '`':	pfmt(f, "`%t", c0);
+	case '`':	pfmt(f, "`%t%t", c0, c1);
 	break;
 	case ANDAND:	pfmt(f, "%t && %t", c0, c1);
 	break;
--- /sys/src/cmd/rc/syn.y	Fri Dec  6 05:20:16 2019
+++ /sys/src/cmd/rc/syn.y	Fri Dec  6 05:20:12 2019
@@ -83,7 +83,8 @@
 |	'"' word		{$$=tree1('"', $2);}
 |	COUNT word		{$$=tree1(COUNT, $2);}
 |	WORD
-|	'`' brace		{$$=tree1('`', $2);}
+|	'`' brace		{$$=tree2('`', (struct tree*)0, $2);}
+|	'`' word brace		{$$=tree2('`', $2, $3);}
 |	'(' words ')'		{$$=tree1(PAREN, $2);}
 |	REDIR brace		{$$=mung1($1, $2); $$->type=PIPEFD;}
 keyword: FOR|IN|WHILE|IF|NOT|TWIDDLE|BANG|SUBSHELL|SWITCH|FN