[vim/vim] fix(cli): parsing bug - require exact match for longopt name (PR #21088)

**Problem**

```sh
 # socket arg is parsed wrongly as positional argument buffer(file) arg: 1
gvim --servername=gvim --clientserver socket
```

```sh
/usr/bin/vim -Es --servername=test --clientserver socket \
+'redir! > /dev/stdout' \
+'echo "servername:" .. v:servername' \
+'echo "initialbufname:" .. bufname()' +'qall!'

servername:
initialbufname:socket
```

**Solution**
Use exact string comparison for `servername`, `serversend`, and
`clientserver` instead of prefix comparison, so trailing characters are not
accepted as part of those option names.
You can view, comment on, or merge this pull request online at:

  https://github.com/vim/vim/pull/21088

-- Commit Summary --

  * cli(fix): require exact match for longopt name

-- File Changes --

    M src/main.c (6)
    M src/testdir/test_startup.vim (12)

-- Patch Links --

https://github.com/vim/vim/pull/21088.patch
https://github.com/vim/vim/pull/21088.diff

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