Slightly modernize realpath(3). SUSv4 requires that implementation returns EINVAL if supplied path is NULL, and ENOENT if path is empty string [1]. Bring prototype in conformance with SUSv4, adding restrict keywords. Allow the resolved path buffer pointer be NULL, in which case realpath(3) allocates storage with malloc(). MFC r206898: Free() is not allowed to modify errno, remove safety brackets around it. Add small optimization, do not copy a string to the buffer that is to be freed immediately after. MFC r206997: Move realpath(3) prototype to a POSIX section. MFC r206998: Add standards section, improve wording, taking into account the handling of NULL and changed type in declaration.