Improve doc on std::os::setenv for Unix.
Improve doc on std::os::split_paths for Unix
Improve doc on std::os::pipe for Window and Unix

Add a precision about the return type.
Refactoring and add an example for std::os::self_exe_name.
Refactoring and add an example for std::os::self_exe_name.
Refactoring and adding an Example to std::os::homedir.

Conflicts:
	src/libstd/os.rs
fixup! Fix useless repetition.
fixup! Precision about handling an error.
fixup! getenv key variable unused.