Remove enable_extended_FILE_stdio()

Even on Illumos it's only available in the 32-bit programming
environment, and, quoth enable_extended_FILE_stdio(3C):

Historically, 32-bit Solaris applications have been limited to using
only the file descriptors 0 through 255 with the standard I/O
functions (see stdio(3C)) in the C library. The extended FILE
facility allows well-behaved 32-bit applications to use any
valid file descriptor with the standard I/O functions.

where "well-behaved" means that it

does not directly access any fields in the FILE structure pointed
to by the FILE pointer associated with any standard I/O stream,

And the stdio/flush.c implementation reads:

/*
 * if this is not an internal extended FILE then check
 * if _file is being changed from underneath us.
 * It should not be because if
 * it is then then we lose our ability to guard against
 * silent data corruption.
 */
if (!iop->__xf_nocheck && bad_fd > -1 && iop->_magic != bad_fd) {
    (void) fprintf(stderr,
        "Application violated extended FILE safety mechanism.\n"
        "Please read the man page for extendedFILE.\nAborting\n");
    abort();
}

This appears to be an insane workaround for broken implementation with
exposed FILE internals and _file being an u8, both only on non-LP64;
it's shimmed out on all LP64 targets in Illumos,
and we shim it out as well: just get rid of it

This appears to've been originally fixed in illumos-gate
a5f69788de7ac07553de47f7fec8c05a9a94c105 ("PSARC 2006/162 Extended FILE
space for 32-bit Solaris processes", "1085341 32-bit stdio routines
should support file descriptors >255"), which also bears extendedFILE
and enable_extended_FILE_stdio(3C):

• unsigned char _file; /* UNIX System file descriptor */ + unsigned char _magic; /* Old home of the file descriptor */ + /* Only fileno(3C) can retrieve the value now */

and

+/*
+ * Macros to aid the extended fd FILE work.
+ * This helps isolate the changes to only the 32-bit code
+ * since 64-bit Solaris is not affected by this.
+ */
+#ifdef  _LP64
+#define        GET_FD(iop)             ((iop)->_file)
+#define        SET_FILE(iop, fd)       ((iop)->_file = (fd))
+#else
+#define        GET_FD(iop)             \
+               (((iop)->__extendedfd) ? _file_get(iop) : (iop)->_magic)
+#define        SET_FILE(iop, fd)       (iop)->_magic = (fd); (iop)->__extendedfd = 0
+#endif

Also remove the 1k setrlimit(NOFILE) calls: that's the default on Linux,
with 64k on Illumos and 171k on FreeBSD

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13411