rc: introduce automatically quoted mini syntax

This is an experimental change that propose an explicit mini syntax
that automatically quote arguments like

 --foo=bar
 -DMACRO=baz
 if=file of=file

This mini-syntax is enabled by a single '$' that marks the end of
variable assignments in a command. The scope of the syntax is a
single command: it's automatically turned of on &, &&, || and so on.

The price of this sugar is that you can't define variables after the
$ marker till the end of the command.

Some examples:

% a=1 echo b=$a
rc: #d/0: token '=': syntax error
% a=1 $ echo b=$a
b=1
% $ eval prefix=$home/foo && echo $prefix
/usr/glenda/foo
% $ eval prefix=$home/foo; echo ./configure --prefix=$prefix
rc: #d/0: token '=': syntax error
% $ eval prefix=$home/foo; $ echo ./configure --prefix=$prefix
./configure --prefix=/usr/glenda/foo
% inf=/dev/random out=/dev/null $ echo dd if=$inf of=$out
dd if=/dev/random of=/dev/null

Also read on 9fans http://marc.info/?t=149366703800002&r=1&w=2

sys/src/cmd/rc/rc.h

@@ -67,6 +67,7 @@ struct tree{
 tree *newtree(void);
 tree *token(char*, int), *klook(char*), *tree1(int, tree*);
 tree *tree2(int, tree*, tree*), *tree3(int, tree*, tree*, tree*);
+tree *treeeq(int, tree*, tree*);
 tree *mung1(tree*, tree*), *mung2(tree*, tree*, tree*);
 tree *mung3(tree*, tree*, tree*, tree*), *epimung(tree*, tree*);
 tree *simplemung(tree*), *heredoc(tree*);

sys/src/cmd/rc/syn.y

@@ -18,6 +18,7 @@
 };
 %type<tree> line paren brace body cmdsa cmdsan assign epilog redir
 %type<tree> cmd simple first word comword keyword words
+%type<tree> qcmd qsimple qpareq qfirst qword qcomword
 %type<tree> NOT FOR IN WHILE IF TWIDDLE BANG SUBSHELL SWITCH FN
 %type<tree> WORD REDIR DUP PIPE
 %%
@@ -33,12 +34,31 @@ cmdsan:	cmdsa
 |	cmd '\n'
 brace:	'{' body '}'		{$$=tree1(BRACE, $2);}
 paren:	'(' body ')'		{$$=tree1(PCMD, $2);}
+	/* auto-quoted mini syntax */
+qfirst:	qcomword
+|	qfirst '^' qword	{$$=tree2('^', $1, $3);}
+qword:	keyword			{lastword=1; $1->type=WORD;}
+|	qcomword
+|	qword '^' qword		{$$=tree2('^', $1, $3);}
+qcomword: '$' qword		{$$=tree1('$', $2);}
+|	'$' qword SUB words ')'	{$$=tree2(SUB, $2, $4);}
+|	'"' qword		{$$=tree1('"', $2);}
+|	COUNT qword		{$$=tree1(COUNT, $2);}
+|	WORD
+qpareq:	qword '=' qword		{$$=treeeq('^', $1, $3);}
+qsimple: qfirst
+|	qsimple '{' '}'		{$$=tree2(ARGLIST, $1, token("{}", WORD));}
+|	qsimple qpareq		{$$=tree2(ARGLIST, $1, $2);}
+|	qsimple qword		{$$=tree2(ARGLIST, $1, $2);}
+|	qsimple redir		{$$=tree2(ARGLIST, $1, $2);}
+qcmd:	'$' qsimple		{$$=simplemung($2);}
 assign:	first '=' word		{$$=tree2('=', $1, $3);}
 epilog:				{$$=0;}
 |	redir epilog		{$$=mung2($1, $1->child[0], $2);}
 redir:	REDIR word		{$$=mung1($1, $1->rtype==HERE?heredoc($2):$2);}
 |	DUP
 cmd:				{$$=0;}
+|	qcmd
 |	brace epilog		{$$=epimung($1, $2);}
 |	IF paren {skipnl();} cmd
 				{$$=mung2($1, $2, $4);}

sys/src/cmd/rc/tree.c

@@ -72,6 +72,17 @@ tree3(int type, tree *c0, tree *c1, tree *c2)
 	return t;
 }
 
+tree*
+treeeq(int type, tree *c0, tree *c1)
+{
+	char *old;
+	old = c0->str;
+	c0->str = smprint("%s=", old);
+	c0->quoted = 1;
+	free(old);
+	return tree2('^', c0, c1);
+}
+
 tree*
 mung1(tree *t, tree *c0)
 {