runtime: Move start's --id to a positional parameter

I added this as an option in 5033c59 (Add an --id option to 'start',
2015-09-15), because some callers might want to leave ID generation to
the runtime.  When there is a long-running host process waiting on the
container process to perform cleanup, the runtime-caller may not need
to know the container ID.  However, runC has been requiring a
user-specified ID since [1], and the coming create/start split will
follow the early-exit 'create' from [2], so require an ID here.  We
can revisit this if we regain a long-running 'create' process.

You can create a config that adds no isolation vs. the runtime
namespace or completely joins another set of existing namespaces.  It
seems odd to call that a new "container", but the ID is really more of
a process ID, and less of a container ID.  The "container" phrasing is
just a useful hint that there might be some isolation going on.  And
we're always creating a new "container process" with 'start' (which
will become 'create').

[1]: opencontainers/runc#541
     opencontainers/runc@a7278cad (Require container id as arg1,
     2016-02-08, opencontainers/runc#541)
[2]: opencontainers/runc#827
     Summary: Implement create and start

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

runtime.md

@@ -37,9 +37,9 @@ For example, POSIX systems define [`LANG` and related environment variables][pos
 
 Start a container from a [bundle directory][bundle].
 
+* *Arguments*
+    * *`<ID>`* Set the container ID to create.
 * *Options*
-    * *`--id <ID>`* Set the container ID when creating or joining a container.
-      If not set, the runtime is free to pick any ID that is not already in use.
     * *`--bundle <PATH>`* Override the path to the [bundle directory][bundle] (defaults to the current working directory).
 * *Standard streams:* The runtime MUST attach its standard streams directly to the application process without inspection.
 * *Environment variables*
@@ -51,7 +51,7 @@ Start a container from a [bundle directory][bundle].
 
 ```
 # in a bundle directory with a process that echos "hello" and exits 42
-$ funC start --id hello-1
+$ funC start hello-1
 hello
 
 $ echo $?
@@ -74,7 +74,7 @@ Request the container state.
 
 ```
 # in a bundle directory with a process that sleeps for several seconds
-$ funC start --id sleeper-1 &
+$ funC start sleeper-1 &
 $ funC state sleeper-1
 {
   "ociVersion": "1.0.0-rc1",
@@ -114,7 +114,7 @@ $ echo $?
 
 ```
 # in a bundle directory with a process ignores TERM
-$ funC start --id sleeper-1 &
+$ funC start sleeper-1 &
 $ funC kill sleeper-1
 $ echo $?
 0