[posix] add UART flush and clean up (#5312)

This commit mainly implements the otPlatUartFlush().
- fix code because MSG_NOSIGNAL is not a macro.
- set SO_NOSIGPIPE once.

tests/scripts/expect/cli-big-table.exp

@@ -0,0 +1,78 @@
+#!/usr/bin/expect -f
+#
+#  Copyright (c) 2020, The OpenThread Authors.
+#  All rights reserved.
+#
+#  Redistribution and use in source and binary forms, with or without
+#  modification, are permitted provided that the following conditions are met:
+#  1. Redistributions of source code must retain the above copyright
+#     notice, this list of conditions and the following disclaimer.
+#  2. Redistributions in binary form must reproduce the above copyright
+#     notice, this list of conditions and the following disclaimer in the
+#     documentation and/or other materials provided with the distribution.
+#  3. Neither the name of the copyright holder nor the
+#     names of its contributors may be used to endorse or promote products
+#     derived from this software without specific prior written permission.
+#
+#  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+#  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+#  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+#  ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+#  LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+#  CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+#  SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+#  INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+#  CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+#  ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+#  POSSIBILITY OF SUCH DAMAGE.
+#
+
+source "tests/scripts/expect/_common.exp"
+
+set max_node 15
+set spawn_ids(1) [spawn_node 1]
+set spawn_id $spawn_ids(1)
+expect_after {
+    timeout { exit 1 }
+}
+send "panid 0xface\n"
+expect "Done"
+send "ifconfig up\n"
+expect "Done"
+send "thread start\n"
+expect "Done"
+wait_for "state" "leader"
+expect "Done"
+
+for {set i 2} {$i <= $max_node} {incr i} {
+    set spawn_ids($i) [spawn_node $i]
+    set spawn_id $spawn_ids($i)
+    expect_after {
+        timeout { exit 1 }
+    }
+    send "mode rs\n"
+    expect "Done"
+    send "panid 0xface\n"
+    expect "Done"
+    send "ifconfig up\n"
+    expect "Done"
+    send "thread start\n"
+    expect "Done"
+    wait_for "state" "child"
+    expect "Done"
+}
+
+set spawn_id $spawn_ids(1)
+expect_after {
+    timeout { exit 1 }
+}
+send "child table\n"
+expect "Done"
+
+for {set i 1} {$i <= $max_node} {incr i} {
+    set spawn_id $spawn_ids($i)
+    expect_after {
+        timeout { exit 1 }
+    }
+    dispose
+}