* callback.c [HAVE_LIMITS_H]: Include limits.h.
Include libiberty.h.
(os_close, os_read, os_write, os_fstat, os_ftruncate): Support fd
being either end of a pipe.
(os_pipe, os_pipe_empty, os_pipe_nonempty): New functions.
(os_shutdown): Clear pipe state.
(default_callback): Initialize new members.
* sim-options.c (STANDARD_OPTIONS): New member OPTION_SYSROOT.
(standard_options): Support --sysroot=<path>.
(standard_option_handler): Handle OPTION_SYSROOT.
* syscall.c (simulator_sysroot): Define, initialized empty.
(get_path): Prepend simulator_sysroot to absolute file path.
[HAVE_STRING_H]: Include string.h.
[!HAVE_STRING_H && HAVE_STRINGS_H]: Include strings.h.
* nrun.c [HAVE_UNISTD_H]: Include unistd.h.
(main): If simulator_sysroot is not empty, chdir there.
* sim-config.h (simulator_sysroot): Declare.
(os_fstat): Likewise. Validate fd argument.
(cb_host_to_target_stat): Delete big_p arg. If HS arg is NULL,
just compute target stat struct length.
* syscall.c: #include "libiberty.h", <sys/types.h>, <sys/stat.h>.
(ENOSYS,ENAMETOOLONG): Provide definitions if missing.
(get_string): Return host errno values so they can be properly
translated later.
(cb_syscall): Likewise.
(cb_syscall, cases open,unlink): Use get_path instead of get_string.
(cb_syscall, case read): Use read_stdin for file descriptor 0.
(cb_syscall, case write): Use write_stderr for file descriptor 2.
(cb_syscall): Add cases for lseek, unlink, stat, fstat, time.
(get_path): New function.
