Loosen the processing of *_IF_aliasN vars to be less strict. Previously,
the first alias had to be _alias0 and processing stopped at the first non-
defined variable (preventing gaps). Allowing gaps gives the administrator
the ability to group aliases in an adhoc manner and also lifts the
requirement to renumber aliases simply to comment-out an existing one.
Aliases are processed in numerical ascending order.
NB: Also Patches mdconfig{,2} rc(8) boot scripts to loosen the numbering
scheme for mdconfig_mdN settings to be less strict in the same manner.
Discussed on: -rc
4f557eb938