kernel: stat/fsstat/isatty syscalls, and O_APPEND

Four things userland had no way to ask for:

- SYS_STAT: type, link count, size and inode for a path. The trap needs HL
  for its dispatch table, so a call taking both a path and an output buffer
  passes a request block - the shape SYS_NET already established. usr/stat.h
  wraps it, header-only like sock.h, because the block must live in the
  caller's RAM.
- SYS_FSSTAT: blocks/inodes, total and free. The bitmaps are already cached
  write-through in fs_bmbuf, so this is the popcount boot_fs already prints.
- SYS_ISATTY: is this process's stdin the console? The shell needs it to tell
  an interactive session from a script.
- O_APPEND: open a file positioned at EOF. There is no seek syscall, so this
  is the only way to add to a file - without it nothing running ON the Game
  Boy can build a multi-line file, shell scripts included.

Together these back ls -l, df, and `>>`.

1 files changed, 6 insertions, 2 deletions

diff --git a/usr/gbos.h b/usr/gbos.h
index 5fd897a..014aea4 100644
--- a/usr/gbos.h
+++ b/usr/gbos.h
@@ -20,6 +20,9 @@ unsigned char pollin(void);             /* poll OSK for a typed char, or 0 */
 unsigned char pollcon(void);            /* poll console ring (link-injected
                                            input, filled by net pumps), or 0 */
 unsigned char sys_net(void *req);       /* socket layer trap (see sock.h)  */
+unsigned char sys_stat(void *req);      /* stat trap (see stat.h)          */
+unsigned char sys_fsstat(void *buf);    /* free blocks/inodes (see stat.h) */
+unsigned char isatty(void);             /* 1 if stdin is the console       */
 void          gsleep(unsigned char units); /* delay units*(1/64)s (raw trap)  */
 void          gticks(unsigned char buf[4]); /* 64 Hz uptime ticks, 32-bit LE  */
 void          msleep(unsigned int ms);     /* delay ~ms milliseconds          */
@@ -42,8 +45,9 @@ unsigned char mkdir(const char *path);
 unsigned char chdir(const char *path);
 unsigned char opendir(const char *path);
 void          poweroff(void);   /* clean emulator shutdown (saves) */
-#define O_READ  0
-#define O_WRITE 1
+#define O_READ   0
+#define O_WRITE  1
+#define O_APPEND 2 /* write at the end, keeping the existing contents */
 #define NOFD    0xFF
 #define EISDIR  0xFE     /* open() failed: path is a directory */