Correct some parts based on feedback and suggestions by netchild:

• add more explanations about why it is a good idea to remove old libs
• move the "make delete-old" step after the second mergemaster, because

that is safer [1]

• remove the extra "make check-old-libs" because it is done together with

"make check-old"

• BATCH_DELETE_OLD_FILES is a make variable and not an environment variable
• warn more vividly that the user has to recompile ports and other programs

before running "make delete-old-libs" and explain how to do this

• mention that portupgrade can also be used to recompile ports

[1] this brings the handbook in sync with the description in
/usr/src/UPDATING changed in r216591.

Discussed with: netchild