* lib/commonio.c: Avoid PATH_MAX. On glibc, we can use realpath

	* lib/commonio.c: Avoid PATH_MAX. On glibc, we can use realpath
	with a NULL argument.
	* src/useradd.c: Replace PATH_MAX by a fixed constant. The buffer
	was not meant as a storage for a path.
	* src/useradd.c, src/newusers.c, src/chpasswd.c: Better detection
	of fgets errors. Lines shall end with a \n, unless we reached the
	end of file.
	* libmisc/copydir.c: Avoid PATH_MAX. Support file paths with any
	length. Added readlink_malloc().