strdup.3: Function appeared in 4.3BSD-Reno, not 4.4BSD

Linux claims 4.3BSD, we claim 4.4BSD and OpenBSD claims 4.3BSD-Reno. It turns
out that OpenBSD got it right: the function was added in late 1988 a few months
after 4.3BSD-Tahoe, well in advance of 4.3BSD-Reno.

Reviewed by: bcr
Approved by: bcr
Differential Revision: https://reviews.freebsd.org/D27392