pkg/aflow: detect patch subsystem/To/Cc lists

[dvyukov]

Probably as the final step of patch generation we need to figure out the patch subsystem, To/Cc lists (is something else necessary to mail a fix?).
Subsystem may affect some email formatting/mailing rules. E.g. [PATCH net] for net subsystem, or we may have different mailing/moderation rules per-subsystem.