util-lib: split out allocation calls into alloc-util.[ch]

util-lib: move a number of fs operations into fs-util.[ch]

util-lib: split out fd-related operations into fd-util.[ch] There are more than enough to deserve their own .c file, hence move them over.

util-lib: split our string related calls from util.[ch] into its own file string-util.[ch] There are more than enough calls doing string manipulations to deserve its own files, hence do something about it. This patch also sorts the #include blocks of all files that needed to be updated, according to the sorting suggestions from CODING_STYLE. Since pretty much every file needs our string manipulation functions this effectively means that most files have sorted #include blocks now. Also touches a few unrelated include files.

tree-wide: never use the off_t unless glibc makes us use it off_t is a really weird type as it is usually 64bit these days (at least in sane programs), but could theoretically be 32bit. We don't support off_t as 32bit builds though, but still constantly deal with safely converting from off_t to other types and back for no point. Hence, never use the type anymore. Always use uint64_t instead. This has various benefits, including that we can expose these values directly as D-Bus properties, and also that the values parse the same in all cases.

fileio: consolidate write_string_file*() Merge write_string_file(), write_string_file_no_create() and write_string_file_atomic() into write_string_file() and provide a flags mask that allows combinations of atomic writing, newline appending and automatic file creation. Change all users accordingly.

test: fix test-copy without /etc/os-release.

test-copy: test copy_bytes()

util: rework rm_rf() logic - Move to its own file rm-rf.c - Change parameters into a single flags parameter - Remove "honour sticky" logic, it's unused these days

util: rework strappenda(), and rename it strjoina() After all it is now much more like strjoin() than strappend(). At the same time, add support for NULL sentinels, even if they are normally not necessary.

machined: when cloning a raw disk image, also set the NOCOW flag

copy: use btrfs reflinking only whe we know we copy full files

copy: teach copy_bytes() sendfile() support, and then replace sendfile_full() by it

tests: add test-copy