Rewrite BusyBee to use a new interface.
Reenable busybee_mta
Remove leftover ifdef
Make distcheck pass.
Add a "deliver" function.
Fix an error where BUSYBEE_DISRUPTED returned the wrong server_id
Link with lrt
Add "generate_id"
Add external polling support
Add support for dropping links.
Expose the fd to poll on
Add a way to wake up threads in BusyBee
Add an extra debug statement for mappings.
Add BUSYBEE_EXTERNAL to operator <<
Add a debug message for when we receive messages
Move some static functions out of the classes.

All three busybee_* classes used the same discover/generate_id
functions.  They were moved to the busybee_utils.{h,cc} files.  The
busybee_returncode.h prototypes were in busybee_utils.cc file and were
moved to busybee_returncode.cc
Make busybee signal-safe

For programs that care about signals, BusyBee will use epoll_pwait, and
return BUSYBEE_INTERRUPTED when interrupted.  The program must use
sigprocmask to block all signals while within BusyBee so that they'll be
reliably delivered from the event loop.  Otherwise, signals are
swallowed by BusyBee and one, more, or all threads may block in "recv"
even though there is a signal to handle.
Add a "wakeup" call to wake "recv"ing threads
Fix a double-free
Fix two more double frees.

Same mistake as last time.  Searched for and corrected all cases.