open tls keys/certificate as root, pass fd's to the unprivileged child process

makes it easier to use tls keys/certs managed by other tools, with or without
acme. the root process has access to open such files. the child process reads
the key from the file descriptor, then closes the file.

for issue #30 by inigoserna, thanks!

config/doc.go

@@ -125,6 +125,8 @@ describe-static" and "mox config describe-domains":
 				# (optional)
 				ACME:
 
+				# Key and certificate files are opened by the privileged root process and passed
+				# to the unprivileged mox process, so no special permissions are required.
 				# (optional)
 				KeyCerts:
 					-