Although using -i with -c does not seem very useful, it seems inappropriate
to read commands from the terminal in this case.
Side effect: if the -s -c extension is used and the -s option is turned off
using 'set +s' during the interactive part, the shell now exits after an
error or interrupt. Note that POSIX only specifies -s as option to sh, not
to set.
See also Austin Group issue #718.