Style.

Simplify cleanup on failure in realpath(3).

Fix several buffer overflows in realpath(3), and other minor issues.

PR:	219154

If realpath() is called on pathnames like "/dev/null/." or "/dev/null/..",
it should fail with [ENOTDIR]. Pathnames like "/dev/null/" already failed as
they should.

Also, put the check for non-directories after lstatting the previous
component instead of when the empty component (consecutive or trailing
slashes) is detected, saving an lstat() call and some lines of code.

PR:		kern/82980
MFC after:	2 weeks

Submitted by:	Christoph Mallon <christoph.mallon@gmx.de> (via private mail)

The ENOTDIR mapping was introduced in r235266 for kern/128933 based on
an interpretation of the somewhat ambiguous language in the POSIX realpath
specification.  The interpretation is inconsistent with Solaris and Linux,
a regression from 9.0, and does not appear to be permitted by the
description of ENOTDIR:

     20 ENOTDIR Not a directory.  A component of the specified pathname
             existed, but it was not a directory, when a directory was
             expected.

PR:		standards/171577
MFC after:	3 days

http://austingroupbugs.net/view.php?id=385#c713
(Resolved state) recommend this way for the current standard (called
"earlier" in the text)

"However, earlier versions of this standard did not require this, and the
same example had to be written as:

    // buf was obtained by malloc(buflen)
    ret = write(fd, buf, buflen);
    if (ret < 0) {
        int save = errno;
        free(buf);
        errno = save;
        return ret;
    }
"

from feedback I have for previous commit it seems that many people prefer
to avoid mass code change needed for current standard compliance
and prefer to track unpublished standard instead, which requires now
that free() itself must save errno, not its usage code.

So, I back out "save errno across free()" part of previous commit,
and will fill PR for changing free() isntead.

2) Remove now unused serrno.

MFC after:      1 week

"The setting of errno after a successful call to a function is
unspecified unless the description of that function specifies that
errno shall not be modified."

However, free() in IEEE Std 1003.1-2008 does not mention its interaction
with errno, so MAY modify it after successful call
(it depends on particular free() implementation, OS-specific, etc.).

So, save errno across free() calls to make code portable and
POSIX-conformant.

2) Remove unused serrno assignment.

MFC after:      1 week

[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

The size passed to strlcat() must depend on the input length, not the
output length. Because the input and output buffers are equal in size,
the resulting binary does not change at all.