Assorted C-string -> std::string conversions in network

[rubidium42]

Motivation / Problem

The ongoing conversion of C-string to std::string.

Description

All kinds of small, but scattered in networking code, changes to go from C-strings to std::string.

• Use fmt in the network address creation.
• Use std::string is IsInNetmask; https://godbolt.org/z/n8TTM5osz is a simple sort of unit test for this change.
• Let GetHostName/GetClientIP directly return the std::string reference instead of the C-string, to prevent converting is back to a std::string later on (e.g. in IsInNetmask), but also in other locations.
• Let NetworkError just return a reference to its std::string.
• Use fmt in HTTP request creation and pass the host as std::string reference.
• Use std::string_view for network compatability checks; https://godbolt.org/z/sj9Ya3q6W is a simple unit test for this change.
• Use std::string for returning the client name.
• Use std::string for (temporarily) storing the name of the downloading content.

Limitations

Contains #9372.

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[TrueBrain]

YOLO!

[TrueBrain]

Sure!

[rubidium42]

I'd rather use item->value.value() but... https://twitter.com/timur_audio/status/1184424350105161730?lang=nl