fix do dir returning no $!

fix do dir returning no $!

do()ing a directory was returning false/empty string in $!, which isn't
an error, yet documentation says $! should have the error code in it.
Fix this by returning EISDIR for dirs, and EINVAL for block devices.
[perl #125774]

Remove "errno = 0" and comment added in b2da7ead68, since now there is no
scenario where errno is uninitialized, since the dir and block device
failure branches now set errno, where previously they didn't.