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)