Keep the mnemonic variables from the mnemonic command (closes #80)

The mnemonic command ran with the tool's whole environment, so a
mnemonic set in KEYFUNC_MNEMONIC reached it, although the command is
there to supply one of its own. It now gets the environment ssh and
sftp already get, with KEYFUNC_MNEMONIC and KEYFUNC_MNEMONIC_COMMAND
removed. The function that builds that environment moves from the ssh
command group into internal/mnemonic, beside the two variable names,
so all three programs use the one copy. The README says so.

Model: opus-5-5

README.md

@@ -125,8 +125,9 @@ joined by single spaces, whatever whitespace is around or between them, so one
 word per line, tabs or extra spaces give the same keys.
 
 `KEYFUNC_MNEMONIC` and `KEYFUNC_MNEMONIC_COMMAND` are removed from the
-environment before the system `ssh` (`keyfunc ssh to`) and `sftp`
-(`keyfunc ssh install`) are started, so the mnemonic is never handed on to them.
+environment before the mnemonic command, the system `ssh` (`keyfunc ssh to`) and
+`sftp` (`keyfunc ssh install`) are started, so the mnemonic is never handed on
+to them.
 
 Every command takes `--index` / `-n` and `--mnemonic-command`, and has `--help`.
 `keyfunc --version` prints the version. `make build` stamps it; a binary