command-line-interface: Add a 'kill' command

Partially catch up with be59415 (Split create and start, 2016-04-01,
#384).  The interface is based on POSIX [1], util-linux [2], and GNU
coreutils [3].

The TERM/KILL requirement is a minimum portability requirement for
soft/hard stops.  Windows lacks POSIX signals [4], and currently
supports soft stops in Docker with whatever is behind
hcsshim.ShutdownComputeSystem [5].  The docs I'm landing here
explicitly allow that sort of substitution, because we need to have
soft/hard stop on those platforms but *can't* use POSIX signals.  They
borrow wording from 35b0e9e (config: Clarify MUST for platform.os and
.arch, 2016-05-19, #441) to recommend runtime authors document the
alternative technology so bundle-authors can prepare (e.g. by
installing the equivalent to a SIGTERM signal handler).

[1]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/kill.html
[2]: http://man7.org/linux/man-pages/man1/kill.1.html
[3]: http://www.gnu.org/software/coreutils/manual/html_node/kill-invocation.html
[4]: https://groups.google.com/a/opencontainers.org/forum/#!topic/dev/PlGKu7QUwLE
     Subject: Fwd: Windows support for OCI stop/signal/kill (runtime-spec#356)
     Date: Thu, 26 May 2016 11:03:29 -0700
     Message-ID: <20160526180329.GL17496@odin.tremily.us>
[5]: https://github.com/docker/docker/pull/16997/files#diff-5d0b72cccc4809455d52aadc62329817R230

Signed-off-by: W. Trevor King <wking@tremily.us>

command-line-interface.md

@@ -102,10 +102,46 @@ $ echo $?
 0
 ```
 
+### kill
+
+[Send a signal][kill] to the container process.
+
+* *Arguments*
+  * *`<ID>`* The container being signaled.
+* *Options*
+  * *`--signal <SIGNAL>`* The signal to send (defaults to `TERM`).
+    The runtime MUST support `TERM` and `KILL` signals with [the POSIX semantics][posix-signals].
+    The runtime MAY support additional signal names.
+    On platforms that support [POSIX signals][posix-signals], the runtime MUST implement this command using POSIX signals.
+    On platforms that do not support POSIX signals, the runtime MAY implement this command with alternative technology as long as `TERM` and `KILL` retain their POSIX semantics.
+    Runtime authors on non-POSIX platforms SHOULD submit documentation for their TERM implementation to this specificiation, so runtime callers can configure the container process to gracefully handle the signals.
+* *Standard streams:*
+  * *stdin:* The runtime MUST NOT attempt to read from its stdin.
+  * *stdout:* The runtime MAY print diagnostic messaged to stdout, and the format for those lines is not specified in this document.
+  * *stderr:* The runtime MAY print diagnostic messages to stderr, and the format for those lines is not specified in this document.
+* *Exit code:* Zero if the signal was successfully sent to the container process and non-zero on errors.
+  Successfully sent does not mean that the signal was successfully received or handled by the container process.
+
+#### Example
+
+```
+# in a bundle directory with a process ignores TERM
+$ funC start --id sleeper-1 &
+$ funC kill sleeper-1
+$ echo $?
+0
+$ funC kill --signal KILL sleeper-1
+$ echo $?
+0
+```
+
 [bundle]: bundle.md
+[kill]: runtime.md#kill
+[kill.2]: http://man7.org/linux/man-pages/man2/kill.2.html
 [posix-encoding]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap06.html#tag_06_02
 [posix-lang]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_02
 [posix-locale-encoding]: http://www.unicode.org/reports/tr35/#Bundle_vs_Item_Lookup
+[posix-signals]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/signal.h.html#tag_13_42_03
 [runtime]: glossary.md#runtime
 [start]: runtime.md#start
 [state]: runtime.md#state