filesystem: getting file_type from a directory_entry without causing a system call on linux

[Taru Karttunen <taruti@…>]

When iterating over a directory with directory_iterator the directory_entries are created with the file_type component of the m_symlink_status defined if the filesystem supports it on Linux due to BOOST_FILESYSTEM_STATUS_CACHE.

However the directory iteration does not produce the permission component of the file status. Thus if using somedirectoryentry.symlink_status().type() the library performs a superfluous lstat system call because status_known does not succeed due to permissions_present not succeeding.

This is possible to fix e.g. by adding a symlink_type method to the directory_entry class, a patch implementing it is linked below:

​https://github.com/taruti/filesystem/commit/835bcd3c13697cbd7fc3d8574fd07475eeada398

ps. Is the documentation manually or automatically created and does it need a separate patch?