job: use a lookup table for merging of job types
It is easier to see what job_type_merge() is doing when the merging rules are written in the form of a table. job_type_is_superset() contained redundant information. It can be simplified to a simple rule: Type A is a superset of B iff merging A with B gives A. Two job types are conflicting iff they are not mergeable. Make job_type_lookup_merge() the core function to decide the type merging. All other job_type_*() are just short wrappers around it. They can be inline. test-job-type gives the same results as before. btw, the systemd binary is smaller by almost 1 KB.