dpkg(1): Clarify valid filenames for dpkg.cfg.d/ fragment files

This information is already present in dpkg.cfg(5), but it might not be
obvious one has to look there, so add it too to the main dpkg(1) where
that directory is referenced and a new entry in the FILES section.

Closes: #674674

debian/changelog

@@ -42,6 +42,9 @@ dpkg (1.16.4) UNRELEASED; urgency=low
   * Clarify in dpkg(1) that the «dpkg -l» example only lists installed
     packages, and that to list available packages «dpkg-query --load-avail»
     has to be used instead. Closes: #673305
+  * Clarify also in the dpkg(1) man page (already present in the dpkg.cfg(5)
+    man page) the valid filenames for /etc/dpkg/dpkg.cfg.d/ fragment files.
+    Closes: #674674
 
   [ Updated man page translations ]
   * German (Helge Kreutzmann).

man/dpkg.1

@@ -23,7 +23,7 @@
 .\" You should have received a copy of the GNU General Public License
 .\" along with this program.  If not, see <http://www.gnu.org/licenses/>.
 .
-.TH dpkg 1 "2012-05-01" "Debian Project" "dpkg suite"
+.TH dpkg 1 "2012-06-05" "Debian Project" "dpkg suite"
 .SH NAME
 dpkg \- package manager for Debian
 .
@@ -351,7 +351,8 @@ See \fBdpkg\-query\fP(1) for more information about the following actions.
 .
 .SH OPTIONS
 All options can be specified both on the command line and in the \fBdpkg\fP
-configuration file \fI/etc/dpkg/dpkg.cfg\fP or the files on the configuration
+configuration file \fI/etc/dpkg/dpkg.cfg\fP or fragment files (with names
+matching this shell pattern \(aq[0-9a-zA-Z_-]*\(aq) on the configuration
 directory \fI/etc/dpkg/dpkg.cfg.d/\fP. Each line in the configuration
 file is either an option (exactly the same as the command line option but
 without leading dashes) or a comment (if it starts with a \fB#\fR).
@@ -671,6 +672,9 @@ Cancels a previous \fB\-\-no\-triggers\fP.
 .
 .SH FILES
 .TP
+.I /etc/dpkg/dpkg.cfg.d/[0-9a-zA-Z_-]*
+Configuration fragment files.
+.TP
 .I /etc/dpkg/dpkg.cfg
 Configuration file with default options.
 .TP