Make do even if PATH_MAX isn't defined

As explained in `man realpath(3)`:

> The POSIX.1-2001 standard version of this function is broken by design,
> since it is impossible  to determine  a  suitable  size  for  the  output
> buffer, resolved_path.  According to POSIX.1-2001 a buffer of size PATH_MAX
> suffices, but PATH_MAX need not be a defined constant, and may have to  be
> obtained  using  pathconf(3).  And asking pathconf(3) does not really help,
> since, on the one hand POSIX warns that the result of pathconf(3) may be huge
> and unsuitable for mallocing memory, and on the other hand pathconf(3) may
> return -1 to  signify  that  PATH_MAX  is  not  bounded.   The  re‐ solved_path
> == NULL  feature,  not standardized in POSIX.1-2001, but standardized in
> POSIX.1-2008, allows this design problem to be avoided.

So we can either not compile, or be pragmatic, and define PATH_MAX to a sane
value, like 4096, which is the one used on Linux and some/most BSD.

This commit also adds two tests to ensure that things aren't catastrophically
broken by this change.

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