fix segfault in debug mode

[szoyj]

Pdnsd generated segmentation fault in debug mode(-g or debug=on), it calls pthread_getspecific() through DEBUG_MSG()
before pthread_key_create() has been called yet. This works on glibc and
uclibc but segfaults on musl because motion is relying on undefined
behaviour here.

References:
openwrt/packages#1847

中文：
pdnsd 在debug模式(启动时加-g或conf里加debug=on)下一启动就直接coredump，用gdb发现最后调用函数是c库函数pthread_getspecific()，google了一圈，最后发现跟 openwrt/packages#1847 是同一个问题，所以就把pthread_key_create提到了main函数的开头。