_ctypes: use_errno=True for ctypes.get_errno()
dohtml: fix DOCDESTTREE path join

When DOCDESTTREE begins with a leading slash, it caused os.path.join()
to misbehave since commit 3f67cb453ee1b74894c230459b43855db7fcc410.
EAPI="5-progress": Support case-insensitive matching of extensions in unpack().