bees: move usage message out of source file and fix a few inaccuracies

It's a pain to read, edit, and format large blocks of text in C++ code,
so rip the usage message out of bees.cc and put it in a plain text file.
Use a minimal translator to convert it into a C string.

While we're here, remove the multiple roots feature from the command
line synopsis, as we don't really support it any more.  Also clarify
that "id 5" is "subvol id 5", and describe in one sentence what
workaround-btrfs-send does.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

src/bees.h

@@ -866,6 +866,7 @@ public:
 
 // And now, a giant pile of extern declarations
 extern int bees_log_level;
+extern const char *BEES_USAGE;
 extern const char *BEES_VERSION;
 string pretty(double d);
 void bees_sync(int fd);