Use sysctl() != -1 instead of !sysctl() to check if

Use sysctl() != -1 instead of !sysctl() to check if
the function call succeeded.

NetBSD 4.0 returns positive values on success, but
NetBSD Current and FreeBSD return zero. OpenBSD's
man page doesn't tell what sysctl() returns on
success. All these BSDs return -1 on error.

Thanks to Robert Elz and Thomas Klausner.