cmd/shfmt: replace -m with -ln=mksh
cmd/shfmt: replace -m with -ln=mksh

Keep -p as a shorthand for -ln=posix, as it's a very common one and the
flag was present in 1.X.

This will mean that adding extra languages won't mean adding more flags.

Fixes #108.

mvdan/sh,mvdan/sh,mvdan/sh,mvdan/sh