Asynchronous/Advanced Input/Output

Changes for 4.81

• work around a bug in musl w.r.t to O_SEARCH and use EIO_O_PATH instead, vertifying the semantics against the subset of O_PATH and O_SEARCH (based on a report by Steve Dondley).
• aio_copy now tries to preallocate the destination file.
• fexecve configure test used the wrong püointer type (patch by Paul Howarth).