Problem: One test in t0005-exec.t uses spaces instead of a tab
for indentation.

Fix tabbing for consistency.

Problem: In the near future additional code flux-exec will use the
zlistx_t data structure.  Some other code in flux-exec already uses
the zhashx_t data structure.  One lingering data structure still
uses the zlist_t data structure.

For consistency going forward, convert the lingering use of zlist_t
to zlistx_t.

Problem: libsubprocess now supports stdin flow control via credits,
but that is not used in flux-exec.

Support credits and flow control in flux-exec to avoid overflowing
the stdin buffer.

Fixes flux-framework#4572

Problem: sdexec does not yet support flow control

Disable flow control if the service is set to sdexec.
Add a --stdin-flow=on|off hidden option to force it either way.

Problem: There is no coverage to ensure that stdin flow control
truly works with flux-exec.

Add a test to t0005-exec.t that sends a 5 meg file while only using
a 4K buffer.  A buffer overflow would almost certainly happen
if flow control was not working.