"FLOGF!" is supposed to treat its first argument as a format
string (but doesn't because that part isn't implemented currently).
That means running something like
```rust
FLOGF!(term_support, "curses var", var_name, "=", value);
```
That would rightly just print "curses var", ignoring the other
arguments.
By contrast, FLOG! is the literal "just join these as a string"
version.
- Add test to verify piped string replace exit code
Ensure fields parsing error messages are the same.
Note: C++ relied upon the value of the parsed value even when `errno` was set,
that is defined behaviour we should not rely on, and cannot easilt be replicated from Rust.
Therefore the Rust version will change the following error behaviour from:
```shell
> string split --fields=a "" abc
string split: Invalid fields value 'a'
> string split --fields=1a "" abc
string split: 1a: invalid integer
```
To:
```shell
> string split --fields=a "" abc
string split: a: invalid integer
> string split --fields=1a "" abc
string split: 1a: invalid integer
```
Most of it is duplicated, hence untested.
Functions like mbrtowc are not exposed by the libc crate, so declare them
ourselves.
Since we don't know the definition of C macros, add two big hacks to make
this work:
1. Replace MB_LEN_MAX and mbstate_t with values (resp types) that should
be large enough for any implementation.
2. Detect the definition of MB_CUR_MAX in the build script. This requires
more changes for each new libc. We could also use this approach for 1.
Additionally, this commit brings a small behavior change to
read_unquoted_escape(): we cannot decode surrogate code points like \UDE01
into a Rust char, so use � (\UFFFD, replacement character) instead.
Previously, we added such code points to a wcstring; looks like they were
ignored when printed.
