Provide variable definition.

[AdrienDebrie]

Example: sequencer ping oberon[104-109] --define noexec='--noexec' --define trace='--Debug'.

Note however, that both --noexec and --Debug can be default variable initialized by the sequencer itself (for example variables predefined by the sequencer can be prefixed by %opt_ hence %opt_noexec and %opt_trace).

Other variables: %opt_dbname.

Those variables are available in filters, actions and depsfinders. The name will be %varname (empty if undefined).