spawn: permit using IOBuffer as stdout (#52461)
People expect to use this (the docs even almost even suggested it at
some point), so it is better to make it work as expected (and better
than they can emulate) than to criticize their choices.
Also fix a few regressions and handling mistakes in setup_stdios:
- #44500 tried to store a Redirectable into a SpawnIO, dropping
FileRedirect
- CmdRedirect did not allocate a ProcessChain, so it would call
setup_stdio then call setup_stdios on the result of that, which is
strongly discouraged as setup_stdio(s) should only be called once
- BufferStream was missing `check_open` calls before writing, and
ignored `Base.reseteof` as a possible means of resuming writing after
`closewrite` sends a shutdown message.
- Add `closewrite` to more methods, and document it.
Fixes #39311
Fixes #49234
Fixes #49233
Fixes #46768
