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.
45003aa79a