fadvise: new module providing a simpler interface to posix_fadvise

fadvise: new module providing a simpler interface to posix_fadvise

* bootstrap.conf: Include the new module
* gl/lib/fadvise.c: Provide a simpler interface to posix_fadvise.
(fadvise): Provide hint to the whole file associated with a stream.
(fdadvise): Provide hint to the specific portion of a file
associated with a file descriptor.
* gl/lib/fadvise.h: Redefine POSIX_FADV_* to FADVISE_* enums.
* gl/modules/fadvise: New file.
* m4/jm-macros.m4: Remove the no longer needed posix_fadvise check.
* .x-sc_program_name: Exclude test-fadvise.c from this check.
* gl/tests/test-fadvise (main): New test program.
* gl/modules/fadvise-testss: A new index to reference the tests.
* src/sort.c (stream_open): Use the new interface.
* src/dd.c (iwrite): Likewise.