MFC r206893:

MFC r206893:
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.

git-svn-id: svn://svn.freebsd.org/base/stable/8@207599 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f