do_lookup return values and error code need to change.

Code analysis fixes

See merge request !1

* When include C headers extern "C" tells that here comes a C header, no suprises.

* Using range based for avoid useless temporary variables.

* use const ref when iterating over dir entries.
* use type alias bfs:: is more convinient
* replace a last c-style cast by static_cast

* nullptr provides more type safetieness.

* Add != 0 when working on ints.

* Use boost version 1.62
* Boost is now required - fail if not found.
* Break too long line.

First cut on dentry support and permissions. First proof-of-concept implementation for opendir, readdir, lookup, and releasedir. Implement next after paper is done.
dentry support is only partial and has to be restructured as the implementation of highlevel fuse is not transferable to lowlevel fuse. Readdir is significantly more complex but also much more lightweight which will help performance.

Private data of fuse is still used for inode and will be moved later to the singleton.

getattr() implemented, read and write metadata is back online, major restructuring of how the userdata is accessed.
File system paths are now accessible from anywhere without fuse_req_t being required. Fields might become somewhat const in the future.
spdlogger is now available systemwide.

adafs_ll_init, adafs_ll_destroy, adafs_ll_getattr first cut with low-level API. Metadata only for root and temporary.