trap (_Msh_POSIXtrap()): better temp file atomicity
trap (_Msh_POSIXtrap()): better temp file atomicity

Better error check for atomic creation of temporary file (use 'is -L dir
/tmp && can write /tmp' instead of just 'can traverse /tmp'; note that for
directories 'can write' implies 'can traverse').

Code cleanup: no longer read the temporary file back into a variable, but
just source it directly as a dot script.

Make the decidion on whether to background 'rm -f' based on whether job
control is active (m) instead of whether the shell is interactive (i).
(Same change in harden() for determining the SIGPIPE exit status.)

modernish/modernish