refactor: prepare the interactive shell in its own module

The prompt-hook step (#41) pushed linux.py past its line count in the
quality baseline. The three steps that make netmiko's fresh shell fit to
read - history off, prompt hooks off, no line wrap - and the prompt
pattern move to napalm_linux/_shell.py, under strict mypy. linux.py
shrinks below where it was; the baseline follows.

napalm_linux/_shell.py

@@ -0,0 +1,54 @@
+"""The interactive shell netmiko opens, made fit to be read before the first command."""
+
+from __future__ import annotations
+
+import logging
+from typing import Any
+
+logger = logging.getLogger(__name__)
+
+#: The end of a command's output when nothing better is known: a line that looks
+#: like a shell prompt.
+PROMPT_RE = r"[#$\>]\s*$"
+
+#: Sent before anything else in the interactive session (NetOrk/netork#1039):
+#: the shell writes no history file and records nothing more. The leading space
+#: keeps this line itself out of a history under ``HISTCONTROL=ignorespace``;
+#: a shell without ``set -o history`` (dash) only complains, to /dev/null.
+HISTORY_OFF = " unset HISTFILE; set +o history 2>/dev/null; true"
+
+#: Sent right after ``HISTORY_OFF`` (#41): systemd 258's
+#: ``/etc/profile.d/80-systemd-osc-context.sh`` wraps every command in OSC 3008
+#: sequences from ``PS0`` and ``PROMPT_COMMAND``, each with a fresh UUID. It appends
+#: to ``PROMPT_COMMAND`` as an array, so only ``unset`` removes its hook.
+PROMPT_HOOKS_OFF = " unset PS0 PROMPT_COMMAND; true"
+
+#: Keeps the PTY from wrapping long output lines (e.g. docker JSON).
+NO_LINE_WRAP = "stty cols 10000 2>/dev/null || true"
+
+
+def prepare_shell(device: Any, hostname: str) -> None:
+    """Make netmiko's fresh shell *device* fit to read, one step at a time.
+
+    A step the shell refuses is logged and skipped: none of them is worth
+    failing the connection for.
+    """
+    # No history before anything else is typed (NetOrk/netork#1039).
+    try:
+        device.send_command(HISTORY_OFF, expect_string=PROMPT_RE)
+    except Exception:
+        logger.debug("Could not switch the shell history off on %s", hostname, exc_info=True)
+
+    # No systemd OSC 3008 context around every command (#41). netmiko learned
+    # the prompt with those sequences in front, and a prompt carrying a fresh
+    # UUID never matches again, so it reads the prompt anew once they are gone.
+    try:
+        device.send_command(PROMPT_HOOKS_OFF, expect_string=PROMPT_RE)
+        device.set_base_prompt()
+    except Exception:
+        logger.debug("Could not switch the prompt hooks off on %s", hostname, exc_info=True)
+
+    try:
+        device.send_command(NO_LINE_WRAP, expect_string=PROMPT_RE)
+    except Exception:
+        logger.debug("%s: ignored", hostname, exc_info=True)