test-subprocess.cpp: reindent
Add fd_handle::mkstemp()
Add subprocess::redirect_to()
subprocess: do not use O_CLOEXEC

O_CLOEXEC descriptors used in file actions are properly passed to the
child even with O_CLOEXEC.
fd_handle::dup() now sets O_CLOEXEC
pg_testdb: redirect server output to a log file