MFC r312118,r312121:

MFC r312118,r312121:

r312118:

Fix -Wformat issue with zero-length format string passed to err(3)

Tested with:	clang, gcc 4.2.1, gcc 4.9

r312121:

Follow up to r312118

State that execve failed instead of just printing out the program name
and strerror(errno) via err(3).

git-svn-id: svn://svn.freebsd.org/base/stable/10@312320 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f