Refactor $*KERNEL internal use of uname

[ugexe]

Uses nqp::uname() on MoarVM instead of shelling out, while shelling out on other backends.

Changes .release() and .version() to return more appropriate values -- uname version instead of uname release for .version(), and uname release instead of uname version for .release() 😕 By making these consistent with their uname counterpart people can better understand the values it may contain and by what name.

[ugexe]

It would be nice if we could make .version() return a Str instead of a Version. If anything .release() should be a Version, but even then it doesn't entirely make sense to create the version object since the Version comparison operators on strings may not order themselves as Version expects.

[lizmat]

Thank you for your work and persistence :-)

[lizmat]

Perhaps it makes sense to create a StrVersion allomorph?