* path.h: Make PATH_SYMLINK an alias for MOUNT_SYMLINK.
* syscalls.cc (stat_worker): Use extension search mechanism in path_conv to
look for .exe rather than trying to special case it here.
* mount.h: Make MOUNT_SYMLINK a real option.
* path.h (class path_conv): Add a char * operator for the most common case.
* syscalls.cc (_unlink): Rewrite to use FILE_FLAG_DELETE_ON_CLOSE when possible
(i.e., on NT).
