In StringUtil.h, the lambdas wrapping `Common::ToLower(char)` and `Common::ToUpper(char)` were only necessary due to the function names being overloaded.

IniFile: Migrate `Common::CaseInsensitiveLess` to StringUtil

With 12 uses of `JoinStrings` in the codebase vs 36 uses of `fmt::join`, fmtlib's range adapter for string concatenation with delimiters is clearly the preferred option.

Migrating `Common::CaseInsensitiveLess` to StringUtil.h will hopefully discourage rolling one's own solution in the future for case-insensitive associative containers when this (quite robust!) solution already exists.

`Common::CaseInsensitiveStringCompare::IsEqual` was removed in favor of using the `Common::CaseInsensitiveEquals` function.

The `a.size() != b.size()` condition in `Common::CaseInsensitiveEquals` can be removed, since `std::ranges::equal` already checks this condition (confirmed in libc++).

fmt::join was moved into fmt/ranges.h

Signed-off-by: Alfred Wingate <parona@protonmail.com>

NetBSD doesn't put packages in /usr/local like /CMakeLists.txt thought.
The `#ifdef __NetBSD__` around iconv was actually breaking compilation
on NetBSD when using the system libiconv (there's also a GNU iconv
package)
A C program included from C++ source broke on NetBSD specifically, work
around it.

This doesn't fix compilation on NetBSD, which is currently broken, but
is closer to correct.