sched/signal: Add support for SIGINT in addition to SIGKILL. drivers/serial: Use SIGINT instead of SIGKILL when control-C is pressed.

Documentation/NuttShell.html

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       <h1><big><font color="#3c34ec"><i>NuttShell (NSH)</i></font></big></h1>
-      <p>Last Updated:  August 22, 2018</p>
+      <p>Last Updated:  August 28, 2018</p>
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 <p><small>
   <b>NOTE</b>:
-  NuttX does not support a FULL POSIX signalling system.
-  Standard signals like SIGCHLD, SIGINTR, SIGKILL, etc. do not exist in NuttX and sending those signals may not have the result that you expect.
+  NuttX does not support a FULL POSIX signaling system.
+  A few standard signal names like <code>SIGCHLD</code>, <code>SIGUSR1</code>, <code>SIGUSR2</code>, <code>SIGALRM</code>, and <code>SIGPOLL</code> exist in the system.
+  However, they do not have the default actions that you might expect.
   Rather, NuttX supports only what are referred to as POSIX real-time signals.
   These signals may be used to communicate with running tasks, may be use to waiting waiting tasks, etc.
-  But, as an example, <code>kill -9</code> (SIGKILL) will not terminate a task.
+</p>
+<p>
+  If the configuration option <code>CONFIG_SIG_DEFAULT</code> is enabled, then default actions for the <code>SIGINT</code> and <code>SIGKILL</code> signals (only) will be supported.
+  In that case, as an example, <code>kill -9</code> (SIGKILL) will, indeed, terminate a task.
+  Caution should be exercised, however, because this is likely to cause memory leaks and to strand resource since there is insufficient clean-up in certain build configurations.
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