runtime: Add --pid-file to 'create'

You can get the PID by calling 'state' [1], and container PIDs may not
be portable [2].  But it's a common way for interfacing with init
systems like systemd [3], and for this first pass at the command line
API folks are ok with some Linux-centrism [4].

[1]: opencontainers/runtime-spec#511 (comment)
     Subject: Add initial pass at a cmd line spec
[2]: opencontainers/runtime-spec#459
     Subject: [ Runtime ] Allow for excluding pid from state
[3]: http://ircbot.wl.linuxfoundation.org/meetings/opencontainers/2016/opencontainers.2016-07-20-21.03.log.html#l-69
[4]: http://ircbot.wl.linuxfoundation.org/meetings/opencontainers/2016/opencontainers.2016-07-13-17.03.log.html#l-39

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

runtime.md

@@ -41,6 +41,7 @@ For example, POSIX systems define [`LANG` and related environment variables][pos
     * *`<ID>`* Set the container ID to create.
 * *Options*
     * *`--bundle <PATH>`* Override the path to the [bundle directory][bundle] (defaults to the current working directory).
+    * *`--pid-file <PATH>`* The runtime MUST write the container PID to this path.
 * *Standard streams:*
     * *stdin:* The runtime MUST NOT attempt to read from its stdin.
     * *stdout:* The handling of stdout is unspecified.