Add a function to export vars that bsd.port.mk generates from fork/exec.

This will be useful in Poudriere to avoid needless fork/exec for every
port when gathering dependencies.

Example usage:

MAKE=make sh -c '. Mk/Scripts/functions.sh; export_index_env; export PACKAGE_BUILDING=1; truss -f make -C sysutils/zfstools -V BUILD_DEPENDS 2>&1' | grep exec

This eliminates 14 exec/fork calls for this example, when PACKAGE_BUILDING
is also set during -V.

Care should be taken with UID not being passed down into actual builds as it
may conflict with non-root builds.

With hat: portmgr