termux: declarative ~/.ssh/config + termux-api package

- home/termux.nix: programs.ssh.settings with the 7 known hosts
  (replaces hand-copied ~/.ssh/config; ssh aliases z-s/z-st/z-o/z-ot
  removed, lamet/pubray-1 kept since they have no Host entry)
- modules/termux/termux-api.nix: build termux-api 0.59.1 (cmake,
  am resolved from PATH, shebangs fixed); adds termux-battery-status,
  termux-notification, termux-clipboard-*, etc. Needs the Termux:API
  Android app (com.termux.api from F-Droid) as the actual backend
- mobile.nix: enable android-integration.am (termux-am backend for am)

modules/termux/default.nix

@@ -86,5 +86,10 @@ in
       #!${pkgs.runtimeShell}
       exec ${pkgs.openssh}/bin/sshd -f /etc/ssh/sshd_config -D "$@"
     '')
+
+    # termux-battery-status, termux-notification, termux-clipboard-*, ...
+    # Needs the Termux:API Android app (com.termux.api, from F-Droid) as the
+    # actual backend; see termux-api.nix.
+    (pkgs.callPackage ./termux-api.nix { })
   ];
 }