Fix sysop reported problem:

9/7 13:58:21 evnt BINKPOLL Locking ../fido/out.26a/00c80001.bsy
9/7 13:58:21 evnt BINKPOLL Error 2 creating ../fido/out.26a/00c80001.bsy

The directory ../fido/out.26a did not exist.

lock_flow() now uses mkpath() to create all the directories it needs to
create the .bsy file.