Define MAXPATHLEN to be at least PATH_MAX, if that's defined. Python uses

Define MAXPATHLEN to be at least PATH_MAX, if that's defined. Python uses
MAXPATHLEN-sized buffers for various output-buffers (like to realpath()),
and that's correct on BSD platforms, but not Linux (which uses PATH_MAX, and
does not define MAXPATHLEN.) Cursory googling suggests Linux is following a
newer standard than BSD, but in cases like this, who knows. Using the
greater of PATH_MAX and 1024 as a fallback for MAXPATHLEN seems to be the
most portable solution.