tang-show-keys: Probe for curl, add documentation

It's needed by the tang-show-keys program.

debian/control

@@ -51,6 +51,8 @@ Depends: ${misc:Depends}, ${shlibs:Depends},
     adduser,
     jose,
     libjansson4,
+Suggests:
+    curl,
 Description: network-based cryptographic binding server - common files
  Tang is a service for binding cryptographic keys to network presence.
  It offers a secure, stateless, anonymous alternative to key escrow

debian/patches/debian/2024-05-31.hint-curl.patch

@@ -0,0 +1,31 @@
+Subject: tang-show-keys: Probe for curl first
+Author: Christoph Biedl <debian.axhn@manchmal.in-ulm.de>
+Date: 2024-05-31
+Forwarded: not-needed
+
+--- a/doc/tang-show-keys.1.adoc
++++ b/doc/tang-show-keys.1.adoc
+@@ -24,6 +24,9 @@
+ <PORT> is the port that the Tang server is listening on on localhost.
+ It defaults to 80.
+ 
++This program requires curl. It might be necessary to install that
++beforehand.
++
+ == AUTHOR
+ 
+ Marius Vollmer <marius.vollmer@redhat.com>
+--- a/src/tang-show-keys
++++ b/src/tang-show-keys
+@@ -25,6 +25,11 @@
+     exit 1
+ fi
+ 
++if ! command -v curl >/dev/null; then
++    echo "Fatal: The curl program is needed but not installed"
++    exit 1
++fi
++
+ port=${1-80}
+ 
+ if test -n "$2"; then

debian/patches/series

@@ -10,3 +10,4 @@ debian/2021-04-19.non-usrmerged.patch
 debian/2021-09-30.use-var-lib.patch
 debian/2021-09-30.xinetd-support.patch
 debian/2023-07-04.test-timeout.patch
+debian/2024-05-31.hint-curl.patch