New installs wrongly use smtp binaries for lmtp

New installs of postfix 3.1.4 on Zesty have broken LMTP configurations by default.

In /etc/postfix/master.cf, lmtp is called using the smtp binary, instead of the lmtp binary.

lmtp unix - - y - - smtp

However, according to Postfix, "The process name, "smtp" or "lmtp", determines the protocol". From https://git.launchpad.net/postfix/tree/src/smtp/smtp.c?h=stable/v3.1#n1330

This is fixed in the Debian upstream in 3.1.4-7 (and by extension, Artful), but should probably be backported into Zesty as it breaks LMTP functionality, and Zesty is considered stable.