LinuxKPI: string.h implement memcpy_and_pad()

Add a memcpy variant which takes length of source and destination
buffers and a padding character in case there is free space in the
destination. This is used by a wireless driver.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D37226

(cherry picked from commit 6a647ae51409cd56cc4a78d0a64ba3ff3aa3aac0)