bktr: Use memset(0) instead of zero initialization for struct sigaction

bktr: Use memset(0) instead of zero initialization for struct sigaction

sigaction is not defined in standards as a struct starting with another
struct. Some *BSD variants do however, resulting in a warning from the
zero initialization, which this change eliminates.

This partially reverts a92be9b856bd11b081041c43c25d442028fe9a63.