Fix getcwd: return char* (POSIX) instead of int

Fix getcwd: return char* (POSIX) instead of int

ulibc getcwd() returned int (0 on success, -1 on error) but POSIX
specifies char* (buf on success, NULL on error). Bash and other
ported software call getcwd expecting a pointer return; getting 0
(NULL) on success caused:

  shell-init: error retrieving current directory: getcwd:
  cannot access parent directories: Bad address

The kernel syscall already returns 0 on success, so the fix is in
the ulibc wrapper: return buf on success, NULL on error (setting
errno). This matches the newlib wrapper in posix_stubs.c which had
the correct signature all along.

Updated all internal callers (sh, pwd, realpath) from int-style
checks (>= 0 / < 0) to pointer-style checks (truthy / !ptr).