PR18115: use begin probe to initialize conditions

PR18115: use begin probe to initialize conditions

We previously initialized the cond_enabled field by evaluating the
conditions during systemtap_module_init(). However, these conditions may
create tmpvars during unparsing (e.g. maps), which require a context to
operate. Therefore, we instead create a synthetic begin probe which will
serve to initialize all cond_enabled fields (or more precisely, switch
them to 0 from their default of 1 if the condition is false).

As a bonus, this allows us to re-use emit_probe_condition_update() for
both condition initialization and updating, and thus to get rid of
emit_probe_condition_initialize().