Allow caching symlinks in kernel page cache. (#551)

Allow caching symlinks in kernel page cache. (#551)

This commit defines a new capability called `FUSE_CAP_CACHE_SYMLINKS`.
It is off by default but you can now enable it by setting this flag in
in the `want` field of the `fuse_conn_info` structure.

When enabled, the kernel will save symlinks in its page cache,
by making use of the feature introduced in kernel 4.20:
https://github.com/torvalds/linux/commit/5571f1e65486be025f73fa6aa30fb03725d362a2