Upgrade mutt_str_append_item() to use struct Buffer

[flatcap]

mutt_str_append_item() appends a string to another string, with a separator character.
This means resizing the original string.

This function would be simpler if it used struct Buffer.
Its callers could then use buf_pool_get() and buf_pool_release().

Most callers use the string and discard it.
The two marked with a red dot need a copy.

Source: gv svg

[roccoblues]

I started this here: #4159

[flatcap]

Done: eb40606