- make "debug.h" a private header so that the DEBUG() macro does not
interfere with Xcode Debug builds, which set DEBUG=1
- move all #include "debug.h" from headers into .c files
- move 'struct idebug' into a new public header "idebug.h" so that
"log.h" can be public
- move HTTPD port number defs from "httpd.h" to "constants.h", so that
"httpd.h" can be private and "conf.h" can be public
- add missing nested includes so each header compiles stand-alone
without error
- #include "sodium.h" instead of <sodium.h>
- #include "sqlite3.h" instead of <sqlite3.h>
- add header guard to "fifo.h"
- fix header guard in "sync_keys.h"
