[9front] sysproc: raise limit on shebang lines, handle quoting.

[ori@eigenstate.org]

when experimenting with auth/box, I realized that the size of
the shebang lines we were using was rather limited.

this raises the limit, but also switches to tokenizing the
command line arguments via tokenize(2), rather than hand
rolled whitespace splitting -- which gives us proper handling
of quotes; now I can box a shell script with:

	#!/bin/auth/box -r/bin -r/usr/git -r/lib -r/mnt rc

diff 90f47fadf8808d84d9bde8316f3945e60650093d uncommitted
--- a/sys/src/9/port/sysproc.c
+++ b/sys/src/9/port/sysproc.c
@@ -243,35 +243,17 @@
 }
 
 static int
-shargs(char *s, int n, char **ap)
+shargs(char *s, int n, char **ap, int nap)
 {
+	char *p;
 	int i;
 
 	if(n <= 2 || s[0] != '#' || s[1] != '!')
 		return -1;
-	s += 2;
-	n -= 2;		/* skip #! */
-	for(i=0;; i++){
-		if(i >= n)
-			return 0;
-		if(s[i]=='\n')
-			break;
-	}
-	s[i] = 0;
-
-	i = 0;
-	for(;;) {
-		while(*s==' ' || *s=='\t')
-			s++;
-		if(*s == 0)
-			break;
-		ap[i++] = s++;
-		while(*s && *s!=' ' && *s!='\t')
-			s++;
-		if(*s == 0)
-			break;
-		*s++ = 0;
-	}
+	if((p = memchr(s+2, '\n', n-2)) == nil)
+		return 0;
+	*p = 0;
+	i = tokenize(s+2, ap, nap-1);
 	ap[i] = nil;
 	return i;
 }
@@ -303,8 +285,8 @@
 	struct {
 		Exec;
 		uvlong	hdr[1];
-	} ehdr;
-	char line[sizeof(ehdr)];
+	} *ehdr;
+	char line[128], buf[128];
 	char *progarg[sizeof(line)/2+1];
 	volatile char *args, *elem, *file0;
 	char **argv, **argp, **argp0;
@@ -353,22 +335,23 @@
 		if(!indir)
 			kstrdup(&elem, up->genbuf);
 
-		n = devtab[tc->type]->read(tc, &ehdr, sizeof(ehdr), 0);
+		n = devtab[tc->type]->read(tc, buf, sizeof(buf), 0);
 		if(n >= sizeof(Exec)) {
-			magic = beswal(ehdr.magic);
+			ehdr = (void*)buf;
+			magic = beswal(ehdr->magic);
 			if(magic == AOUT_MAGIC) {
 				if(magic & HDR_MAGIC) {
-					if(n < sizeof(ehdr))
+					if(n < sizeof(*ehdr))
 						error(Ebadexec);
-					entry = beswav(ehdr.hdr[0]);
-					text = UTZERO+sizeof(ehdr);
+					entry = beswav(ehdr->hdr[0]);
+					text = UTZERO+sizeof(*ehdr);
 				} else {
-					entry = beswal(ehdr.entry);
+					entry = beswal(ehdr->entry);
 					text = UTZERO+sizeof(Exec);
 				}
 				if(entry < text)
 					error(Ebadexec);
-				text += beswal(ehdr.text);
+				text += beswal(ehdr->text);
 				if(text <= entry || text >= (USTKTOP-USTKSIZE))
 					error(Ebadexec);
 
@@ -393,8 +376,8 @@
 		/*
 		 * Process #! /bin/sh args ...
 		 */
-		memmove(line, &ehdr, n);
-		n = shargs(line, n, progarg);
+		memmove(line, buf, n);
+		n = shargs(line, n, progarg, nelem(progarg));
 		if(n < 1)
 			error(Ebadexec);
 		/*
@@ -411,8 +394,8 @@
 
 	t = (text+align) & ~align;
 	text -= UTZERO;
-	data = beswal(ehdr.data);
-	bss = beswal(ehdr.bss);
+	data = beswal(ehdr->data);
+	bss = beswal(ehdr->bss);
 	align = BY2PG-1;
 	d = (t + data + align) & ~align;
 	bssend = t + data + bss;