sys.mk: Introduce WORLD_CFLAGS.

sys.mk: Introduce WORLD_CFLAGS.

Main purpose of this flag is to provide user a way to safely add extra
compiler flags to the build system without accidentally removing things
like -pipe or -Ox flags. Also it will help to ensure that none of user
set world flags will propagate to dports compilation to cause ruckus.
CFLAGS is still available as before provided it will be handled with care.

While there, adjust make.conf(5) to promote WORLD_CFLAGS usage instead of
manipulating CFLAGS directly. Also fix typo, ld.gold is default.