Rationalize documentation.

This commit fixes discrepancies between the 'README' and the command-line help.
Remove notion of cflags vs. ldflags.

The distinction between cflags and ldflags is somewhat arbitrary - end users
should simply care about the flags they must supply to 'c++'.  Furthermore,
'defs' is more in line with how upstream BDE treats flags that should be seen
by dependents.
Provide help for 'def' commands.