Check for use of sh -exc instead of just sh -c

[dsheets]

-x is useful to find out later what was actually executed.
-e is necessary for errors to be propagated correctly in all cases.

Some care will be required to ensure that the false positive rate is not too high due to variations in sh command line invocations.

[dsheets]

Also, consider -u for erroring on using unset variables.

[hannesm]

should maybe sh be similar magic to make (which turns into gmake on FreeBSD), thus people can use sh (not "sh") in opam, and opam expands it to sh -excu!?

[dsheets]

Unfortunately, it is difficult to distinguish between expressions and scripts as arguments to sh and therefore to toggle -c but I am interested in making this easier to use correctly.