Disable strict bourne shell mode to get ksh behaviour in (S)hell

Disable strict bourne shell mode to get ksh behaviour in (S)hell

Enable features like arithmatic expression and thus make the debug aid
`FUNCS_ONLY=1 . /install.sub' work in those prompts.

'!' answers spawn their own ksh process, but (S)hell drops into /.profile
sourced by /bin/sh executed by init(8), i.e. a korn shell defaulting to
strict bourne shell mode.

Shell options in this interactive parent shell do not effect install.sub
aka. /install, /autoinstall and /upgrade as it is executed in a child.

OK afresh1