Fix allocation limit for getcwd syscall

[Roland McGrath]

We have bigger problems if PATH_MAX is defined but is not actually the
limit on the path name length as POSIX specifies.  If there is a larger
limit, that should be the PATH_MAX value.  If there is no limit, then
PATH_MAX should not be defined.

That said, I don't think the code should presume what the kernel
implementation does in this regard.  It should do something other than
crash when it gets ERANGE.