config/interface_modify.sh: before calling a script check if it exists and is executable

config/interface_modify.sh: before calling a script check if it exists and is executable

For non bash shells $_s_script might end with '/*'.

We do the workarround this way, because it makes sense to check
that a script is executable, before trying to execute it.

metze

[ This actually applies to any shell -- Rusty Russell ]
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>