FileID: implement `FileID` struct

`FileID` should encapsulate `st_dev` and `st_ino`. It is a new abstraction
used to check if two files are the same. `FileID`s will be embeded inside
other struts like `buf_t` or `ff_visited_T`, where a full `FileInfo` would be
to big.
FileID: remove `INVALID_DEVICE_ID` macro
FileID: refactor buffer.c to use `FileID`
FileID: refactor ex_cmds2.c to use `FileID`
FileID: refactor file_search.c to use `FileID`
FileID: refactor if_cscope.c to use `FileID`
FileID: refactor path.c to use `FileID`
FileID: refactor os_unix.c to use `FileID`
FileID: make `os_stat()` static
FileID: remove last use of `st_ino` in memline.c

* FileID can’t be used in memline.c, because the block0 is defined to
use only a 32bit ino.
* implemented `os_file_info_get_inode`
* deprecated `os_file_info_get_inode
FileID: remove `CHECK_INODE` macro
Merge #775 'Implement `FileID` struct'