man: Document DPKG_MAINTSCRIPT_PACKAGE environment variable

debian/changelog

@@ -35,6 +35,8 @@ dpkg (1.15.4) UNRELEASED; urgency=low
     (/etc/dpkg/dpkg.cfg.d and /etc/dpkg/dselect.cfg.d respectively).
   * Define DPKG_MAINTSCRIPT_ARCH on the maintainer script environment to the
     architecture the package got built for.
+  * Document DPKG_MAINTSCRIPT_PACKAGE maintainer script environment variable
+    in dpkg man page.
 
   [ Raphael Hertzog ]
   * Replace install-info by a wrapper around GNU's install-info. The wrapper

man/dpkg.1

@@ -610,6 +610,10 @@ The program \fBdpkg\fP will execute when starting a new shell.
 Sets the number of columns \fBdpkg\fP should use when displaying formatted
 text. Currently only used by \-l.
 .TP
+.B DPKG_MAINTSCRIPT_PACKAGE
+Defined by \fBdpkg\fP on the maintainer script environment to the
+package name being handled.
+.TP
 .B DPKG_MAINTSCRIPT_ARCH
 Defined by \fBdpkg\fP on the maintainer script environment to the
 architecture the package got built for.