external backends are all in libexec, use the same define
Merge branch 'master' into portable

Conflicts:
	smtpd/smtpd.h
Merge branch 'portable' of ssh.poolp.org:/git/opensmtpd into portable
define PATH_LIBEXEC
woops i broke stuff
Merge branch 'master' into portable
Merge branch 'portable' of ssh://ssh.poolp.org/git/opensmtpd into portable