buck2: stdin: retry EINTR

Summary:
On Mac you can get this if you CTRL+Z Buck2 while it's running then resume it.
I'm not sure why the signal gets delivered to this thread there and not on
Linux, but here it's clearly not wrong to retry EINTR so we might as well just
do so.

Reviewed By: lmvasquezg

Differential Revision: D47511075

fbshipit-source-id: 6b9dba6f1ad75a40342531a9e2b22ca26e07efb7

Simple refactor of prepare_cd_exe

Summary: Reduce duplication a little bit, make it easier to change things in the next diffs

Reviewed By: ianlevesque

Differential Revision: D47471859

fbshipit-source-id: 715a747cf8d3d0f29c02caa6c825323b0dbe2949

Label global

Summary: Can be used as type.

Reviewed By: ndmitchell

Differential Revision: D47163560

fbshipit-source-id: 73562cd6107af679cdbefca19bc818495c4baa1b