[krbdev.mit.edu #3181] init_ctx.c has goto labels crossing outside of switch arms

[Sam Hartman via RT]

the copy_subkey label in the switch statement that decides what gssapi
algorithm we are using needs to be factored out.

Currently if you remove the des arm of the switch, the rest of the
switch statement breaks because there is a goto back into that block.

Please factor this out of the switch statement.