- 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)

MFC r273929:

r273929 (by jmmv):

MFV: Import atf-0.21.

MFV: Import atf-0.21.

Undefine HAVE_VSNPRINTF_IN_STD.

Should fix the build with g++/libstdc++.

This is what we used to do prior the import of atf 0.20 and the build worked
just fine with both libstdc++ and libc++. Still investigating how to properly
fix this upstream so that we do not hit the same issue on the next import.

MFV: Import atf-0.20.

MFV: Import atf-0.18.

Approved by: rpaulo (mentor)

Add headers created by running configure.