illumos gate

With the changes to isatty() in #15220, a successful call to the printf(3C) functions will now set errno to 25 (ENOTTY) if the output is not a terminal.
This is inconsistent with previous behaviour, and POSIX

#include <stdio.h>
#include <errno.h>

int
main(void)
{
        errno = 123;
        (void) printf("test\n");
        printf("errno: %d\n", errno);
}

%~/isatty
test
errno: 123

% ~/isatty | cat
test
errno: 25

% dtrace -n 'pid$target::isatty:entry{ustack()}' -c ./isatty
dtrace: description 'pid$target::isatty:entry' matched 1 probe
test
errno: 123
dtrace: pid 26325 has exited
CPU     ID                    FUNCTION:NAME
  2  99403                     isatty:entry
              libc.so.1`isatty
              libc.so.1`_findbuf+0x13f
              libc.so.1`_wrtchk+0x61
              libc.so.1`puts+0x8c
              isatty`main+0x18
              isatty`_start_crt+0x87
              isatty`_start+0x18