stdioext: Add support for musl libc.

stdioext: Add support for musl libc.

* m4/fbufmode.m4 (gl_FUNC_FBUFMODE): Test whether __fbufsize exists.
* lib/fbufmode.c (fbufmode): Add conditional code for musl.

* m4/fseterr.m4: New file.
* lib/fseterr.h (fseterr): Define as an alias of __fseterr if that
function exists.
* modules/fseterr (Files): Add m4/fseterr.m4.
(configure.ac): Invoke gl_FUNC_FSETERR. Compile fseterr.c if
__fseterr does not exist.
(Makefile.am): Remove fseterr.c from lib_SOURCES.

* lib/freadable.h: Update comment.

* lib/fwritable.h: Update comment.

* lib/freading.h: Update comment.

* lib/fwriting.h: Update comment.

* m4/freadahead.m4: New file.
* lib/freadahead.h (freadahead): Define as an alias of __freadahead if
that function exists.
* modules/freadahead (Files): Add m4/freadahead.m4.
(configure.ac): Invoke gl_FUNC_FREADAHEAD. Compile freadahead.c if
__freadahead does not exist.
(Makefile.am): Remove freadahead.c from lib_SOURCES.

* m4/freadptr.m4: New file.
* lib/freadptr.h (freadptr): Define as an alias of __freadptr if that
function exists.
* modules/freadptr (Files): Add m4/freadptr.m4.
(configure.ac): Invoke gl_FUNC_FREADPTR. Compile freadptr.c if
__freadptr does not exist.
(Makefile.am): Remove freadptr.c from lib_SOURCES.

* m4/freadseek.m4: New file.
* lib/freadseek.c (freadptrinc): Use __freadptrinc if that function
exists.
* modules/freadseek (Files): Add m4/freadseek.m4.
(configure.ac): Invoke gl_FUNC_FREADSEEK.

* lib/fpurge.c (fpurge): Update comment.

Reported by and with help from Rich Felker <dalias@aerifal.cx>.