individual port, introduce a new variable for bsd.port.mk, VMEM_WARNING.
By setting this variable to `Yes', the user is given a warning about the
virtual memory requirements just before the pre-build stage.
o Also give some examples for different shells for what to do about
it, and mention login.conf(5). bash, for example, has a confusing
-v flag to ulimit.
o Document this in bsd.port.mk.5
espie@ ok
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