cf-dc-api.pl: make source and target optional (use the first recognized_source and repolist element respectively)

Conflicts:
	tools/test/Makefile
cf-dc-api.pl: when install source is not given, try all recognized_sources

Conflicts:
	howto/dc_api.md
#339: do not allow non-hashref parameter sets and check for them more carefully
	#339: do not allow non-hashref parameter sets, to depth 2
cf-dc-api.pl: add new 'choice' API parameter
file_integrity: use new "choice" API option