test: replace sprintf() with snprintf()

There is zero risk of overflowing the buffer, it's just a good habit.
test: replace strcpy() with strncpy()
openbsd: fix uv_ip6_addr() unused variable warnings
openbsd: fix dlerror() const correctness warning
openbsd: fix uv_fs_sendfile() unused variable warnings
build: disable parallel automake tests

Our tests are not parallelizable and besides, they're one big test
from the perspective of the automake test driver anyway. Turn on
serial mode.