Add test for ioctl(SIOCGIFCONF) and polyfill on BSDs

- Use nullness checks when calling weakly linked functions.

- Avoid typedef for reasons described in Linux Kernel style guide.

- Avoid enum in in Windows headers. Earlier in Cosmo's history all one
  hundred files in libc/nt/enum/ used to be enums and it resulted in
  gigabytes of DWARF data almost as large as everything else in the
  codebase combined.

- Bitfields aren't our friends. They have frequent ABI breakages,
  inconsistent arithmetic across compilers, and different endianness
  between cpus. Compiler authors also haven't invested much roi into
  making bit fields go fast so they produce poor assembly.

- Use memccpy() instead of strncpy() or snprintf() for length-bounded
  copying of C strings. strncpy() is a misunderstood function and
  snprintf() is awesome but memccpy() deserves more love.

libc/calls/ioctl_tcsets.c

@@ -0,0 +1,48 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2020 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/calls/internal.h"
+#include "libc/calls/struct/metatermios.internal.h"
+#include "libc/calls/termios.internal.h"
+#include "libc/dce.h"
+#include "libc/sysv/consts/termios.h"
+#include "libc/sysv/errfuns.h"
+
+int ioctl_tcsets_nt(int, uint64_t, const struct termios *);
+
+static int ioctl_tcsets_sysv(int fd, uint64_t request,
+                             const struct termios *tio) {
+  union metatermios t;
+  return sys_ioctl(fd, request, __termios2host(&t, tio));
+}
+
+/**
+ * Changes terminal behavior.
+ *
+ * @see tcsetattr(fd, TCSA{NOW,DRAIN,FLUSH}, tio) dispatches here
+ * @see ioctl(fd, TCSETS{,W,F}, tio) dispatches here
+ * @see ioctl(fd, TIOCGETA{,W,F}, tio) dispatches here
+ */
+int ioctl_tcsets(int fd, uint64_t request, const struct termios *tio) {
+  if (!tio) return efault();
+  if (!IsWindows()) {
+    return ioctl_tcsets_sysv(fd, request, tio);
+  } else {
+    return ioctl_tcsets_nt(fd, request, tio);
+  }
+}