Historically snap-confine used two modes of operation. Initially on core
systems snap-confine would unshare the mount namespace and preform some
alterations but would otherwise run with the contents of the initial
mount namesapce. On classic systems snap-confine would pivot into the
base snap (core) and perform some more alterations.

With the introduction of multiple boot bases, per-snap bases some of
that logic seems rusty. To make it less confusing what is going on
reduce the number of places that determine if legacy (sans-pivot) or
normal mode (with pivot) to exactly one.

There is more cleanup that can be done but it will require making the
invocation structure something we can pass around to more functions.
This will be attempted in subsequent patches.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

This allows us to almost start passing sc_invocation instead of a
plethora of arguments to various functions inside ns-support and
mount-support modules.

This also makes it a tiny bit problematic because is_normal_mode is a
boolean that is only set inside the non-classic code path. This is
correct for now but should be carefully changed later.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>