Make NanoBSD source-able from other scripts.

Summary:
This change converts NanoBSD into a two-script bundle.
- defaults.sh contains all non-CLI code.  Most NanoBSD code is moved into
  this file.
- nanobsd.sh now consists just of a command line interface that calls into
  functions in defaults.sh.

Test Plan: Run NanoBSD using a previously-working configuration.

Reviewers: imp

Reviewed By: imp

Differential Revision: https://reviews.freebsd.org/D1321
The iret instruction may generate #np and #ss fault, besides #gp.
When returning to usermode, the handler for that exceptions is also
executed with wrong gs base.  Handle all three possible faults in the
same way, checking for iret fault, and performing full iret.

Sponsored by:	The FreeBSD Foundation
MFC after:	3 days