Add -w/--write-config to bootstrap a fully-populated config file

Writes every supported config option explicitly -- the six
unshare-* bools defaulted via value_or(true), log-level captured
from the actually active spdlog level (so an explicit --log-level
passed alongside -w is persisted too) -- creating the file and its
parent directory if missing, and prints its full path. Existing
values (including volumes) are preserved untouched.

Also fixes a stale README row left over from unplugging the
namespace probe out of -t/--test.

README.md

@@ -94,7 +94,8 @@ slocker-lite -V|--version
 | `--delete-volume-full <name>` | Like `--delete-volume`, but also recursively deletes the volume's host directory. |
 | `--list-processes` | List running `--run` sessions found by their pid files under `$XDG_STATE_HOME/slocker-lite/run/`, with their pid, container name, and status (`running` or `exited`). |
 | `--clean-processes` | Remove stale pid files (see `--list-processes`) left behind by sessions that are no longer running. |
-| `-t, --test` | Print which `bwrap --unshare-xxx` namespaces the running kernel supports. |
+| `-w, --write-config` | Write a complete config file (creating it, and its parent directory, if missing), filling in every option's current or default value. Useful to bootstrap one for hand-editing. Prints the config file's full path. |
+| `-t, --test` | Run the (currently empty) self-test placeholder. |
 | `--log-level <level>` | Set log verbosity (`trace`, `debug`, `info`, `warn`, `error`, `critical`, `off`). |
 | `-h, --help` | Print usage and exit. |
 | `-V, --version` | Print version information and exit. |
@@ -194,6 +195,14 @@ managed by `-v/--volume` (see above) rather than hand-edited — it's what
 is fine either way (nothing is overridden, and one gets created the first
 time `-v/--volume` is used).
 
+Run `-w/--write-config` to bootstrap a config file: it writes out every
+supported option explicitly (filling in the current or default value for
+anything not already set — so the block above is exactly what a fresh `-w`
+produces), creating the file and its parent directory if they don't exist
+yet, and prints the file's full path. Combine it with other flags to seed
+specific values, e.g. `slocker-lite --log-level debug -w` writes
+`log-level: debug`.
+
 ## How it works
 
 Image layers are imported into `containers-storage` (parent-chained) and the