Add .mailmap for commit author translations

Because it's not possible to enumerate up-front all of the committers to
tmux coming from OpenBSD, at the time a commit is imported in to git from
the OpenBSD CVS repository, the author information is not known to Git,
necessarily.

But it's possible to alter for output the respective author after the fact,
via Git's .mailmap file.  It is this file which will therefore provide a new
mapping of OpenBSD commiter to an actual real name and real email address.

.mailmap

@@ -0,0 +1,21 @@
+Bob Beck 		<beck@openbsd.org> beck <beck>
+Igor Sobrado 		<sobrado@openbsd.org> sobrado <sobrado>
+Jacek Masiulaniec 	<jacekm@openbsd.org> jacekm <jacekm>
+Jason McIntyre 		<jmc@openbsd.org> jcm <jcm>
+Joel Sing		<jsing@openbsd.org> jsing <jsing>
+Marc Espie 		<espie@openbsd.org> espie <espie>
+Matthew Dempsky 	<matthew@openbsd.org> matthew <matthew>
+Matthias Kilian 	<kili@openbsd.org> kili <kili>
+Matthieu Herrb 		<matthieu@openbsd.org> matthieu <matthieu>
+Miod Vallat 		<miod@openbsd.org> miod <miod>
+Nicholas Marriott 	<nicm@openbsd.org> nicm <nicm>
+Okan Demirmen 		<okan@openbsd.org> okan <okan>
+Philip Guenther		<guenther@openbsd.org> guenther <guenther>
+Pierre-Yves Ritschard 	<pyr@openbsd.org> pyr <pyr>
+Ray Lai 		<ray@openbsd.org> ray <ray>
+Ryan McBride		<mcbride@openbsd.org> mcbride <mcbride>
+Stefan Sperling 	<stsp@openbsd.org> stsp <stsp>
+Stuart Henderson 	<sthen@openbsd.org> sthen <sthen>
+Ted Unangst 		<tedu@openbsd.org> tedu <tedu>
+Theo Deraadt 		<deraadt@openbsd.org> deraadt <deraadt>
+William Yodlowsky 	<william@openbsd.org> william <william>