modif: replace --keep-hostname with new --hostname-randomize

Changes:

* Keep hostname by default (same as using `--keep-hostname`)
* Add  `--hostname-randomize` command to randomize the hostname
* Ignore `--keep-hostname` command and print a warning if it is used

Setting a different hostname inside of the sandbox may prevent X11
programs from authenticating to the X server and displaying windows at
all (see #7062).

To avoid breakage, keep the hostname as is by default and only set it to
a random value if a new `hostname-randomize` command is used.

This also avoids potentially surprising behavior, as the user might not
expect the hostname to be changed inside of the sandbox, considering
that usually the protections that are applied firejail involve
restricting access to resources (like file paths), rather than modifying
their values inside of the sandbox.

Fixes #7062

Relates to #7048 #7069.

README.md

@@ -443,15 +443,21 @@ full path or a relative path for the filename.
               Parent is shutting down, bye…
 ```
 
-### --keep-hostname
+### --hostname-randomize
 
 ```text
-       --keep-hostname
-              By default, the hostname is replaced by a random  name  generated
-              by firejail at startup. Use this option to disable this feature.
+       --hostname-randomize
+              Set sandbox hostname to a random value generated by firejail.
+              This is incompatible with --hostname.
 
               Example:
-              $ firejail --keep-hostname
+              $ firejail --hostname-randomize /usr/bin/firefox
+              Note: Changing the hostname may cause breakage related to
+              networking (see #7048
+              <https://github.com/netblue30/firejail/issues/7048>) and may
+              cause X11 programs to crash on startup due to not being able to
+              authenticate to the X server (see #7062
+              <https://github.com/netblue30/firejail/issues/7062>).
 ```
 
 ### Landlock support - ongoing/experimental