patch 9.1.0828: string_T struct could be used more often

patch 9.1.0828: string_T struct could be used more often

Problem:  string_T struct could be used more often
Solution: Refactor code and make use of string_T struct
          for key-value pairs, reformat overlong lines
          (John Marriott)

closes: #15975

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