Added support for .bsy lock files when in BSO/FLO mailer mode

(based on FTS-5005 description) - both detection (won't pack/bundle outgoing
mail for a node that is "busy" with a mailer session) and exclusive-creation
(to inform the mailer not to allow a session for this node while outgoing mail
is being processed).
- Uses strListFind() from updated xpdev.
Debug-level log message for all file deletions.
Some log message update (more detail in some, removal of others).
Bug-fix: outbound FREQ netmail messages would not be deleted after being
converted to BSO *.req files. It appears no one has yet tested this feature?
Cleaned-up sbbsecho.h: use of C99-types, defined SBBSECHO_ARCTYPE_NONE magic
number macro.
Incremented version to 2.32.