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, 16 insertions, 0 deletions

diff --git a/src/syscall.asm b/src/syscall.asm
index 0ed9f85..9e5ca33 100644
--- a/src/syscall.asm
+++ b/src/syscall.asm
@@ -71,6 +71,9 @@ SyscallTable:
     dw sys_sleep                ; 34 SLEEP
     dw sys_pollcon              ; 35 POLLCON
     dw sys_uptime               ; 36 UPTIME
+    dw sys_stat                 ; 37 STAT
+    dw sys_fsstat               ; 38 FSSTAT
+    dw sys_isatty               ; 39 ISATTY
 
 ; -----------------------------------------------------------------------------
 sys_nosys:
@@ -332,6 +335,19 @@ sys_sleep:
     ret
 
 ; -----------------------------------------------------------------------------
+; sys_isatty() -> A = 1 if this process's stdin is the console, 0 if it is a
+; file or a pipe. The shell uses it to tell an interactive session from a
+; script (`sh < file`): no prompt, no echo, and EOF is just the end.
+; -----------------------------------------------------------------------------
+sys_isatty:
+    call CurStdin
+    cp STD_CONSOLE
+    ld a, 0
+    ret nz
+    inc a
+    ret
+
+; -----------------------------------------------------------------------------
 ; sys_exit(code=B) -- become a zombie awaiting reap; never returns.
 ;   - store exit code, set PS_ZOMBIE
 ;   - reparent any children to init (so they can still be waited on)