Add support for -parallel in combination with gfix -icu

[mrotteveel]

gfix -icu rebuilds indexes and thus can be parallelized, but right now it is not possible to use the gfix -parallel option together with -icu as it will result in the message "incompatible switch combination". As such, the -icu option will always use the value of ParallelWorkers in firebird.conf/databases.conf.

Allow use of -parallel in combination with -icu so it is configurable, just like with -sweep.