Fix several buffer overflows in realpath(3).

• The statement "left_len -= s - left;" does not take the slash into account if one was found. This results in the invariant "left[left_len] == '\0'" being violated (and possible buffer overflows). The patch replaces the variable "s" with a size_t "next_token_len" for more clarity.
• "slen" from readlink(2) can be 0 when encountering empty symlinks. Then, further down, "symlink[slen - 1]" underflows the buffer. When slen == 0, realpath(3) should probably return ENOENT (http://austingroupbugs.net/view.php?id=825, https://lwn.net/Articles/551224/).

Some other minor issues:

• The condition "resolved_len >= PATH_MAX" cannot be true.
• Similarly, "s - left >= sizeof(next_token)" cannot be true, as long as "sizeof(next_token) >= sizeof(left)".
• Return ENAMETOOLONG when a resolved symlink from readlink(2) is too long for the symlink buffer (instead of just truncating it).
• "resolved_len > 1" below the call to readlink(2) is always true as "strlcat(resolved, next_token, PATH_MAX);" always results in a string of length > 1. Also, "resolved[resolved_len - 1] = '\0';" is not needed; there can never be a trailing slash here.
• The truncation check for "strlcat(symlink, left, sizeof(symlink));" should be against "sizeof(symlink)" (the third argument to strlcat) instead of "sizeof(left)".

Submitted by: Jan Kokemц╪ller <jan.kokemueller@gmail.com>
PR: 219154
MFC after: 2 weeks