zopen.3: zap file since zopen() is only used via compress(1)
m4(1): Remove two unused manual pages.
share/examples: use .Mt for emails in man pages

While there, get rid of redundant quotes in .Nd lines.
forward.5: use .Ar instead of .Aq for email address

The email is an example and should not be a spurious candidate for
.Mt conversion.  The example below uses .Ar for a local address
already.
tzfile.5: use .In for including tzfile.h

The file resides in /usr/include anyway.
dhclient: partial sync from OpenBSD man pages
Remove some no longer used externs in various system headers.