ash: an unset dynamic variable should not be dynamic

ash: an unset dynamic variable should not be dynamic

Commit b28d4c346 (ash: [VAR] Move unsetvar functionality into setvareq)
dropped the code that caused dynamic variables to lose their special
properties when unset.  Add it back again.

function                                             old     new   delta
setvareq                                             346     360     +14
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(add/remove: 0/0 grow/shrink: 1/0 up/down: 14/0)               Total: 14 bytes

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>