patch 9.1.2044: Inefficient use of ga_concat()

patch 9.1.2044: Inefficient use of ga_concat()

Problem:  Inefficient use of ga_concat()
Solution: Use ga_concat_len() when the length is already known to avoid
          use of strlen() (John Marriott).

Additionally the following changes are done:
os_unix.c:
- in function `socket_server_list_sockets()` use a `string_T` for the
  strings `buf` and `path` for use in `ga_concat_len()`
  and drop un-needed variable `dir`.

quickfix.c:
- in function `qf_jump_print_msg()` use a `string_T` for the string
  `IObuff` for use in `ga_concat_len()`.
- in function `qf_range_text()` use a `string_T` for the string `buf`
  for use in `ga_concat_len()`.

register.c:
- simplify function `execreg_line_continuation()`.

terminal.c:
- in function `read_dump_file()` use a `string_T` for the
  string `prev_char` for use in `ga_concat_len()`.

tuple.c:
- in function `tuple_join_inner()` use a `string_T` for the
  string `s` for use in `ga_concat_len()`. Also, change local struct
  `join_T` to use `string_T`.

vim9type.c:
- in functions `type_name_tuple()` and `type_name_func()`
  use a `string_T` for the string `arg_type` for use in
  `ga_concat_len()`.

closes: #19038

Signed-off-by: John Marriott <basilisk@internode.on.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>