Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

Fix buildworld on FreeBSD 10

Since r336126 we depend on explicit_bzero() for the libmd
bootstrap. Add it to -legacy if it is not found in /usr/include/strings.h.

Reviewed By: ian
Approved By: brooks (mentor)
Differential Revision: https://reviews.freebsd.org/D16245