Fix O_APPEND, a getcwd() kernel panic, and console timestamps

[nrchnnas]

Three small, independent fixes, found while running BusyBox on moss.

1. fs: reg: honour O_APPEND

Bug: OpenFlags::O_APPEND is defined but never checked, so every write goes to the current file position. As a result, shell >> overwrites the file instead of appending:

/ # echo a > /tmp/t; echo b >> /tmp/t; cat /tmp/t
b

Fix: RegFile::write now moves the cursor to the end of the file before each write when the file was opened with O_APPEND. writev goes through write, so it is covered too; pwrite keeps its explicit offset.

Test: the new usertest test_o_append also checks that seeking to 0 first doesn't stop the append.

2. fs: getcwd: return ERANGE when the buffer is too small

Bug: sys_getcwd returns KernelError::TooLarge. That has no errno mapping, so kern_err_to_syscall() reaches todo!(), and any process can panic the kernel with getcwd(buf, 1):

usertest::fs::test_getcwd_buffer_too_small ...[    1.1737690] moss: Kernel panicked at
libkernel/src/error/syscall_error.rs:85:14: not yet implemented: Provided object is too large

Fix: return RangeError (ERANGE), which is what Linux returns in this case.

Test: new usertest test_getcwd_buffer_too_small.

3. console: fix the sub-second field of log timestamps

Bug: the console logger prints uptime.as_micros(), the total microseconds, as the fractional part. After one second it prints [ 1.1737690] (see the panic line above) instead of [ 1.173769].

Fix: use subsec_micros(), matching Linux's printk format.

Testing

• cargo fmt --all -- --check passes, and clippy reports nothing new in the changed files.
• cargo run -r -- --init /bin/usertest prints All tests passed, including both new tests.
• Each new test fails without its fix:
  • the getcwd test panics the kernel (output above);
  • test_o_append fails with left: "third\n\n", right: "first\nsecond\nthird\n".

[hexagonal-sun]

LGTM! I fixed the clippy lint on master. If you rebase your MR should not include that unrelated failure.

[nrchnnas]

Rebased onto master, thanks for the review!