- Copy stable/10@296371 to releng/10.3 in preparation for 10.3-RC1
builds.
- Update newvers.sh to reflect RC1.
- Update __FreeBSD_version to reflect 10.3.
- Update default pkg(8) configuration to use the quarterly branch.

Approved by: re (implicit)

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

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

Don't install private libexecinfo headers

libexecinfo compatibility with devel/libexecinfo port

1. Match shlib number
2. Add libelf dependency

Suggested by: bapt[1]

Add libexecinfo Makefile

Sponsored by: DARPA, AFRL