flux_local_exec() fails with Bad file descriptor on macos

[garlick]

Problem: The most basic subprocess tests fail on macos

./test_subprocess.t
ok 1 - flux_reactor_create
# basic
ok 2 - flux_cmd_create
not ok 3 - flux_local_exec
#   Failed test 'flux_local_exec'
#   at test/subprocess.c line 271.
not ok 4 - subprocess state == RUNNING after flux_local_exec
#   Failed test 'subprocess state == RUNNING after flux_local_exec'
#   at test/subprocess.c line 274.
not ok 5 - flux_local_exec() started pid 4294967295
#   Failed test 'flux_local_exec() started pid 4294967295'
#   at test/subprocess.c line 276.
not ok 6 - flux_subprocess_get_cmd success
#   Failed test 'flux_subprocess_get_cmd success'
#   at test/subprocess.c line 278.
not ok 7 - flux_subprocess_get_reactor success
#   Failed test 'flux_subprocess_get_reactor success'
#   at test/subprocess.c line 280.
not ok 8 - flux_subprocess_get_reactor returns correct reactor
#   Failed test 'flux_subprocess_get_reactor returns correct reactor'
#   at test/subprocess.c line 282.
ok 9 - flux_reactor_run returned zero status
not ok 10 - completion callback called 1 time
#   Failed test 'completion callback called 1 time'
#   at test/subprocess.c line 286.
ok 11 - no file descriptors leaked
1..11
# Looks like you failed 7 tests of 11 run.

Inserting some diag code, it appears that flux_local_exec() in test_basic() fails with

# flux_local_exec: Bad file descriptor
not ok 3 - flux_local_exec
#   Failed test 'flux_local_exec'
#   at test/subprocess.c line 273.