minor, refactor subscription code into sub.[ch]

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
chore, mention pimmel.h where it belongs in libpimmel.la SOURCES

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'code/hygiene' into next

* code/hygiene:
  chore, mention pimmel.h where it belongs in libpimmel.la SOURCES
  minor, refactor subscription code into sub.[ch]