ncp-spinel: Avoid using ssize_t, provide fallbacks for errno defines (#203)
ncp-spinel: Avoid using ssize_t, provide fallbacks for errno defines (#203)

This issue hopefully fully addresses #191. This change addresses two
issues:

* The inappropriate use of `ssize_t`.
* Handling when `EOVERFLOW` and `EINVAL` are not defined by `errno.h`.
jwhui/openthread,nibanks/openthread,georgecpr/openthread,erja-gp/openthread,jjlee9/openthread,bukepo/openthread,erja-gp/openthread,Zolertia/openthread,openthread/openthread,nibanks/openthread,LeZhang2016/openthread,fbsder/openthread,JakubBrachTieto/openthread,mszczodrak/openthread,librasungirl/openthread,jjlee3/openthread,fbsder/openthread,jjlee9/openthread,mszczodrak/openthread,turon/openthread,erja-gp/openthread,vaas-krish/openthread,abtink/openthread,chshu/openthread,librasungirl/openthread,JakubBrachTieto/openthread,jwhui/openthread,aeliot/openthread,openthread/openthread,xiaom-GitHub/openthread,GiedriusM/openthread,gandreello/openthread,Zolertia/openthread,mszczodrak/openthread,LeZhang2016/openthread,srickardti/openthread,chshu/openthread,mszczodrak/openthread,xiaom-GitHub/openthread,JakubBrachTieto/openthread,pvanhorn/openthread,georgecpr/openthread,librasungirl/openthread,gandreello/openthread,pvanhorn/openthread,GiedriusM/openthread,lanyuwen/openthread,nibanks/openthread,librasungirl/openthread,GiedriusM/openthread,abtink/openthread,jjlee3/openthread,lanyuwen/openthread,jjlee3/openthread,JakubBrachTieto/openthread,LeZhang2016/openthread,georgecpr/openthread,lanyuwen/openthread,pvanhorn/openthread,bukepo/openthread,LeZhang2016/openthread,srickardti/openthread,chshu/openthread,xiaom-GitHub/openthread,fbsder/openthread,xiaom-GitHub/openthread,nibanks/openthread,erja-gp/openthread,gandreello/openthread,aeliot/openthread,pvanhorn/openthread,Zolertia/openthread,aeliot/openthread,turon/openthread,jjlee3/openthread,abtink/openthread,fbsder/openthread,chshu/openthread,openthread/openthread,vaas-krish/openthread,jjlee9/openthread,mszczodrak/openthread,fbsder/openthread,jwhui/openthread,lanyuwen/openthread,jjlee9/openthread,aeliot/openthread,mszczodrak/openthread,nibanks/openthread,LeZhang2016/openthread,JakubBrachTieto/openthread,vaas-krish/openthread,fbsder/openthread,Zolertia/openthread,chshu/openthread,JakubBrachTieto/openthread,jjlee9/openthread,lanyuwen/openthread,jwhui/openthread,xiaom-GitHub/openthread,nibanks/openthread,turon/openthread,bukepo/openthread,openthread/openthread,GiedriusM/openthread,pvanhorn/openthread,chshu/openthread,GiedriusM/openthread,pvanhorn/openthread,xiaom-GitHub/openthread,jjlee3/openthread,jjlee9/openthread,Zolertia/openthread,turon/openthread,georgecpr/openthread,gandreello/openthread,erja-gp/openthread,georgecpr/openthread,aeliot/openthread,vaas-krish/openthread,gandreello/openthread,srickardti/openthread,erja-gp/openthread,abtink/openthread,lanyuwen/openthread,gandreello/openthread,georgecpr/openthread,bukepo/openthread,vaas-krish/openthread,aeliot/openthread,srickardti/openthread,LeZhang2016/openthread,vaas-krish/openthread