Removed check for posix version so that missing O_CLOEXEC leads to
compiler error.
Removed extra function since it hat no real purpose anymore.
Error behavior between Windows and Unix now equivalent.
calling log with a string_view as msg called
```
template<typename... Args>
void log(source_loc loc, level::level_enum lvl, string_view_t fmt, const Args &... args)
```
instead of
```
template<class T, typename std::enable_if<std::is_convertible<const T &, spdlog::string_view_t>::value, T>::type * = nullptr>
void log(source_loc loc, level::level_enum lvl, const T &msg)
```
which lead to an unnecessary call to fmt::format
Using the SPDLOG_PREVENT_CHILD_FD option there where still a race when
a other thread was using fork and exec in between the call to fopen and fcntl.
Using open and O_CLOEXEC when possible prevents this race.
I have no idea if this problem exists on Windows.
Ringbuffer sink keeps user-given number of most recent log messages in
memory and returns them upon request (using the ringbuffer_sink::last
method). This can be useful for e.g. remote debugging of a running app.
The meaning of using quotes to #include is implementation defined, so it
may or not may be what we want. At least POSIX
(https://pubs.opengroup.org/onlinepubs/9699919799/utilities/c99.html)
says: "headers whose names are enclosed in double-quotes ( "" ) shall be
searched for first in the directory of the file with the #include line",
so not what we want since "spdlog" ends up twice in the path.
