Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

o style.Makefile(5): remove SRCS.

Add a new program, perror, which takes an errno as a command line argument
and outputs the associated textual message in the same way that
perror(3) would if called within a program.