Remove $FreeBSD$: two-line .h pattern

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

Provide reallocarray() in -legacy, if needed, to allow building head on
previous releases.

Also add a stdlib.h wrapper, which declares the function, otherwise the
compiler may assume it returns int, which can cause segfaults on LP64
architectures.

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D2558