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, 19 insertions, 1 deletions

diff --git a/usr/libc.s b/usr/libc.s
index fbca836..de087b6 100644
--- a/usr/libc.s
+++ b/usr/libc.s
@@ -1,7 +1,7 @@
 	.module libc
 	.globl _writes, _putc, _puts, _nl, _strlen, _readc, _sexit, _getpid, _getargs
 	.globl _open, _close, _fgetc, _fputc, _flist, _fremove, _pipe, _ssend, _srecv, _srecv_nb, _pollin, _pollcon
-	.globl _sys_net, _gsleep, _gticks
+	.globl _sys_net, _gsleep, _gticks, _sys_stat, _sys_fsstat, _isatty
 	;; SDCC sm83 sdcccall(1): 1st arg -> A (byte) or DE (pointer); ret A / BC.
 	;; rst $30 (the trap) clobbers BC/DE/HL; SDCC treats them caller-saved, so
 	;; wrappers need not preserve them - but we compute cleanly regardless.
@@ -318,3 +318,21 @@ _opendir:
 _poweroff:
 	.db	0xED
 1$:	jr	1$
+
+	;; unsigned char sys_stat(void *req /*DE*/) -> A   (see fs.asm sys_stat)
+_sys_stat:
+	ld	c, #37			; SYS_STAT
+	rst	#0x30
+	ret
+
+	;; unsigned char sys_fsstat(void *buf /*DE*/) -> A
+_sys_fsstat:
+	ld	c, #38			; SYS_FSSTAT
+	rst	#0x30
+	ret
+
+	;; unsigned char isatty(void) -> A (1 = stdin is the console)
+_isatty:
+	ld	c, #39			; SYS_ISATTY
+	rst	#0x30
+	ret