Getline touchups.

Getline touchups.
* lib/getdelim.c (getdelim): Revert regression that required *n to
be 0 when *lineptr is NULL.  Preserve errno across funlockfile.
* m4/getdelim.m4 (gl_FUNC_GETDELIM): Check for declaration of
getdelim, rather than whether implementation is missing.
* m4/getline.m4 (gl_FUNC_GETLINE): Likewise for getline.
* lib/stdio_.h (getline): Also declare if replacement is
required.
* doc/functions/getdelim.texi: New file.
* doc/functions/getline.texi: Likewise.
* doc/gnulib.texi (Function Substitutes): Add new files.
Reported by Bruno Haible.