fs: reject open()/read() on a directory (EISDIR)

open() now checks the target's inode type: opening a directory as a file
returns $FE (EISDIR) instead of a valid fd, so 'cat docs' no longer streams the
raw directory block as bytes. cat reports 'is a directory'; wc/head/save treat
any invalid fd (>3) as an open failure. (ls is unaffected - it uses the
structural opendir/list path, not open/getb.)

1 files changed, 2 insertions, 2 deletions

diff --git a/c/head.c b/c/head.c
index e8bf846..3298e54 100644
--- a/c/head.c
+++ b/c/head.c
@@ -16,10 +16,10 @@ void main(void) {
     }
     if (fname) {
         fd = open(fname, O_READ);
-        if (fd == NOFD) { puts("head: no such file"); nl(); return; }
+        if (fd > 3) { puts("head: no such file"); nl(); return; }
     }
     for (;;) {
-        if (fd == NOFD) { char c = readc(); if (c == EOF) break; ch = c; }
+        if (fd > 3) { char c = readc(); if (c == EOF) break; ch = c; }
         else { ch = fgetc(fd); if (ch < 0) break; }
         if (lines < n) putc((char)ch);
         if (ch == '\n') lines++;