use shellcheck on smpirun
rules were mainly about escaping, quoting stuff, getting rid of -a and -o
but also some UUOC https://github.com/koalaman/shellcheck/wiki/SC2002
or performance improvement https://github.com/koalaman/shellcheck/wiki/SC2129
and some trap hint : https://github.com/koalaman/shellcheck/wiki/SC2064
two remainings : DEFAULT_NUMPROCS variable is unused. unclear what it was about, should it be the default num of cores in a generated platform ? or the default -np setting ?
"local" is not posix, but a proposal for now
