libuv: do not require PATH_MAX to be defined

Some platforms (e.g. GNU/Hurd) do not define PATH_MAX.  Add a few other
variants and a fallback constant.  Also use alternatives where possible:

* For readlink(), use lstat() to read the length of the link first.
  If it is not a symlink, report EINVAL before trying to allocate.
  If the size reports as zero, fall back one of the PATH_MAX variants.

* For realpath(), POSIX 2008 allows us to pass a NULL buffer
  to tell it to malloc() internally.

This patch was inspired by downstream patches in Debian packaging
for issues 897061 and 909011.

Issue: #18337

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