tree: Fix various typos and outdated terminology

This was mostly machine generated with the following command:
```
codespell \
    --builtin clear,rare,usage \
    --skip './po/*' --skip './.git/*' --skip './NEWS*' \
    --write-changes .
```
using the latest git version of `codespell` as per [these
instructions](https://github.com/codespell-project/codespell#user-content-updating).

Then I manually checked each change using `git add -p`, made a few
manual fixups and dropped a load of incorrect changes.

There are still some outdated or loaded terms used in GLib, mostly to do
with git branch terminology. They will need to be changed later as part
of a wider migration of git terminology.

If I’ve missed anything, please file an issue!

Signed-off-by: Philip Withnall <withnall@endlessm.com>

gio/gunixmounts.c

@@ -421,7 +421,7 @@ guess_system_internal (const char *mountpoint,
    * to not mengle those mounts with the "regular" mounts (i.e. which points to
    * the root). But because those mounts usually just duplicate other mounts and
    * are completely ignored with mntend-based implementation, let's mark them as
-   * system internal. Given the different approches it doesn't mean that all
+   * system internal. Given the different approaches it doesn't mean that all
    * mounts which were ignored will be system internal now, but this should work
    * in most cases. For more info, see g_unix_mount_get_root_path() annotation,
    * comment in mntent-based _g_get_unix_mounts() implementation and the