runtime: Add a 'kill' command

Similar to the 'signal' command removed in b922732 (Drop exec, pause,
resume, and signal, 2015-12-02, #3).  The
runtime-spec gained a kill operation as part of
opencontainers/runtime-spec@be594153 (Split create and start,
2016-04-01, opencontainers/runtime-spec#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 we're 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 opencontainers/runtime-spec@35b0e9ee (config:
Clarify MUST for platform.os and .arch, 2016-05-19,
opencontainers/runtime-spec#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
     moby/moby@bc503ca8 (Windows: [TP4] docker kill handling,
     2015-10-12, moby/moby#16997)

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

runtime.md

@@ -90,9 +90,45 @@ $ 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 handling of stdout is unspecified.
+    * *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
+```
+
+[kill]: https://github.com/opencontainers/runtime-spec/blob/v1.0.0-rc4/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
 [semver]: http://semver.org/spec/v2.0.0.html
 [standard-streams]: https://github.com/opencontainers/specs/blob/v0.1.1/runtime-linux.md#file-descriptors
 [systemd-listen-fds]: http://www.freedesktop.org/software/systemd/man/sd_listen_fds.html