According to SUSv4, realpath(3) must fail if
[ENOENT] A component of file_name does not name an existing file or

file_name points to an empty string.

[ENOTDIR] A component of the path prefix is not a directory, or the

file_name argument contains at least one non- <slash> character
and ends with one or more trailing <slash> characters and the last
pathname component names an existing file that is neither a
directory nor a symbolic link to a directory.

Add checks for the listed conditions, and set errno accordingly.

Update the realpath(3) manpage to mention SUS behaviour. Remove the
requirement to include sys/param.h before stdlib.h.

PR: 128933
MFC after: 3 weeks