core: turn various execution flags into a proper flags parameter

core: turn various execution flags into a proper flags parameter

The ExecParameters structure contains a number of bit-flags, that were so far
exposed as bool:1, change this to a proper, single binary bit flag field. This
makes things a bit more expressive, and is helpful as we add more flags, since
these booleans are passed around in various callers, for example
service_spawn(), whose signature can be made much shorter now.

Not all bit booleans from ExecParameters are moved into the flags field for
now, but this can be added later.