MFC r173217,176789,176792-176793,176839,176842

MFC r173217,176789,176792-176793,176839,176842

Don't forget to set MAKEFLAGS in the childs' environment
from the .MAKEFLAGS global variable even if it's empty or
unset.  This means setting MAKEFLAGS to just an empty string
in the latter case.

The non-POSIX environment variable MAKE was superseded
by MAKEFLAGS ages ago, so don't mention it in comments.

Update the manpage accordingly and make the whole description of
MAKEFLAGS and .MAKEFLAGS more consistent as this change indeed
brings more consistency into the reliable behaviour of make(1).

Give more details in the manpage on the following topics:
o How global, command-line, and environment variables relate.
o What peculiarities the -f option has WRT ${MAKEFLAGS}.

(.Dd in the manpage is already beyond the date of those changes
in HEAD.)