Sync against taskchampion-sync-server (#3118)

This removes use of gnutls and the TLS implementation, which is no
longer needed (task synchronization is handled via Taskchampion, which
uses `reqwest`, which handles TLS via other Rust dependencies). This
incidentally removes the following config options:
 * `debug.tls`
 * `taskd.ca`
 * `taskd.certificate`
 * `taskd.ciphers`
 * `taskd.credentials`
 * `taskd.key`
 * `taskd.server`
 * `taskd.trust`

src/commands/CmdShow.cpp

@@ -145,7 +145,6 @@ int CmdShow::execute (std::string& output)
     " debug"
     " debug.hooks"
     " debug.parser"
-    " debug.tls"
     " default.command"
     " default.due"
     " default.project"
@@ -193,14 +192,11 @@ int CmdShow::execute (std::string& output)
     " search.case.sensitive"
     " sugar"
     " summary.all.projects"
+    " sync.local.server_dir"
+    " sync.server.client_key"
+    " sync.server.encryption_secret"
+    " sync.server.origin"
     " tag.indicator"
-    " taskd.server"
-    " taskd.ca"
-    " taskd.certificate"
-    " taskd.ciphers"
-    " taskd.credentials"
-    " taskd.key"
-    " taskd.trust"
     " undo.style"
     " urgency.active.coefficient"
     " urgency.scheduled.coefficient"