Add emerge --dynamic-deps <y|n> option.

Add emerge --dynamic-deps <y|n> option.

This makes it possible to disable the dynamic dependency updates that
FakeVartree performs by default.

WARNING: If --dynamic-deps is disabled, then it is necessary to
ensure that an alternative method is used to handle package moves
in dependencies of installed packages. Normally, this is handled
by FEATURES="fixpackages", which is enabled by default and may be
disabled via make.conf(5). Alternatively, in order to manually apply
package moves, run `emaint --fix moveinst` after each emerge --sync
operation (see emaint(1)).