MFC r230786, r246283, r251084, r251085, r251086:

MFC r230786, r246283, r251084, r251085, r251086:

r230786 (imp):
- Allow specification of build shell for the buildenv target.

r246283 (hrs) (partial):
- Add {WORLD,KERNEL}_FLAGS to [BTWK]MAKE.

r251084:
- r245757 introduced warning output if update method is set to
  CVS_UPDATE or SUP_UPDATE.
- CVS exporter for stable/9/ is turned off for nearly one month
  now.
- It is finally time to swing the ax at these update methods.

r251085:
- Fix typo introduced in r251084.

r251086:
- Remove references to CVS_UPDATE and SUP_UPDATE to catch up
  with r251084.

Approved by:	kib (mentor)

git-svn-id: svn://svn.freebsd.org/base/stable/9@252305 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f