Prefer UNIX_PATH_MAX over UNIX_PATH_LEN

Cygwin used to define UNIX_PATH_LEN, but in cygwin-1_7_18-release and
cygwin-64bit-branch, sys/un.h renamed to MAX to follow Linux. (CVS r1.7)

This commit will be pushed using 64-bit ssh-pageant, proving that all is
well here for cygwin64.  For the adventurous, it also does work to let
cygwin64 processes talk to cygwin32 ssh-pageant, and vice versa, as long
as you reconcile their different filesystem mounts in SSH_AUTH_SOCK.