BUILD/MINOR: unix sockets: silence an absurd gcc warning about strncpy()

BUILD/MINOR: unix sockets: silence an absurd gcc warning about strncpy()

Apparently gcc developers decided that strncpy() semantics are no longer
valid and now deserve a warning, especially if used exactly as designed.
This results in issue #304. Let's just remove one to the target size to
please her majesty gcc, the God of C Compilers, who tries hard to make
users completely eliminate any use of string.h and reimplement it by
themselves at much higher risks. Pfff....

This can be backported to stable version, the fix is harmless since it
ignores the last zero that is already set on next line.