(errno): Declare if not defined.
(addext): Work correctly when pathconf returns -1 and leaves
errno alone because there is no limit. Also, work even if
pathconf returns a value greater than SIZE_MAX.
Include <dirname.h> after size_t is defined.
(addext): Use base_len to trim redundant
trailing slashes instead of doing it ourselves.
But do not trim the last slash if it is not redundant.
