Avoid deadlocks when the LOG_LEVEL is set to DEBUG

`fprintf` and `syslog` are thread-safe but not reentrant.
If a signal happen during those functions and the handler calling
them again, the deadlock is form.  Therefore, we need to block signals
until we leave they realm.

https://github.com/nodogsplash/nodogsplash/commit/89c1805d609b73c068fb78d045492103c16e6c74#diff-f5b46886ad081ff6ddf3a5b585346a02

patch by sayuan