Move fill_charp_buf from os to str

[catamorphism]

as per a FIXME in core::os

[kud1ing]

It seems to me that os::as_c_charp() can be replaced with str::as_c_str().
Or is it hat os:c_char is not the same as libc::c_char?

[pnkfelix]

@kud1ing : Nope, AFAICT, os::c_char is the same as libc::c_char (at least that's my interpretation of the use libc::{c_char, ...}; declaration at the top of os.rs.

I suspect the meat of this ticket is getting rid of fill_charp_buf (or migrating it to str). I'll see what happens to the build if I try to drop them.

[lilyball]

fill_charp_buf should probably return an Option<CString> instead of Option<~str>. The only use in libstd is in self_exe_path(), which shouldn't be using ~str (due to #7225)

[brson]

There is no longer anything called charp in libstd. Fixed in passing.