runtime-linux: Condition /proc/self/fd symlinks on source existence

Since 279c3c0 (linux: relax filesystem requirements for container,
2017-01-23, #666) it's no longer guaranteed that /proc will exist.
And there doesn't seem to be much point in requiring symlinks which
will be known broken.

This commit also tightens the timing.  Before it was just "after the
container has `/proc` mounted", which could have happened during the
'delete' operation (if the container authors wanted to be especially
ornery).  With this commit, I've put the creation in step 2 of the
lifecycle.  And within step 2, it happens after 'mounts' has been
processed.

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

runtime-linux.md

@@ -8,7 +8,7 @@ Some of the file descriptors MAY be redirected to `/dev/null` even though they a
 
 ## <a name="runtimeLinuxDevSymbolicLinks" /> Dev symbolic links
 
-After the container has `/proc` mounted, the following standard symlinks MUST be setup within `/dev/` for the IO.
+While creating the container (step 2 in the [lifecycle](runtime.md#lifecycle)), runtimes MUST create the following symlinks if the source file exists after processing [`mounts`](config.md#mounts):
 
 |    Source       | Destination |
 | --------------- | ----------- |