internal/exec: Fix stdio pipe problems

[kevpar]

exec today has two problems with how it handles stdio pipes:

• When Wait completes, closeStdio() is called. This closes the parent-side stdio pipes for receiving IO from the process. This is a problem because once the process has completed, we still need to be able to receive any final output. Today data from the process could be lost because of this.
• The parent's handles to the child-side stdio pipes are not closed after starting the process. Leaving duplicates of these handles in the parent process means that the other ends of the pipes are never closed when the process exits.

This commit makes the following changes:

• The parent's handles to the child-side stdio pipes are now closed after the child is started. This is necessary so that once the child exits, the parent-side pipes will return EOF once the remaining output drains.
• When Wait completes, the parent-side stdio pipes are not closed. The responsibility for this is now left to the client of the exec package. Currently the only user of exec is jobcontainers.JobProcess, which closes handles these when Close is called.

Additionally, the ProcThreadAttributeList is now allocated and used only in Start. Previously it was saved on the Exec object, even though it was not needed elsewhere. This makes the code cleaner, simplifies the Wait logic, and eliminates the chance of leaking memory if an Exec object is GC'd without being Wait'd.