It had the downside of running all the comptime blocks and resolving
all the usingnamespaces of each system, when just trying to discover if
the current system is a particular one.
For Darwin, where it's nice to use `std.Target.current.isDarwin()`, this
demonstrates the utility that #425 would provide.
* `std.os.execve` had the wrong name; it should have been
`std.os.execvpe`. This is now corrected.
* introduce `std.os.execveC` which does not look at PATH, and uses
null terminated parameters, matching POSIX ABIs. It does not
require an allocator.
* fix typo nonsense doc comment in `std.fs.MAX_PATH_BYTES`.
* introduce `std.os.execvpeC`, which is like `execvpe` except it
uses null terminated parameters, matching POSIX ABIs, and thus
does not require an allocator.
* `std.os.execvpe` implementation is reworked to only convert
parameters and then delegate to `std.os.execvpeC`.
* `std.os.execvpeC` improved to handle `ENOTDIR`. See #3415
