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

util-lib: move more file I/O related calls into fileio.[ch]

util-lib: split out user/group/uid/gid calls into user-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: there is no ENOTSUP on linux Replace ENOTSUP by EOPNOTSUPP as this is what linux actually uses.

build-sys: export sd_path APIs

sd-path: add missing header

shared: rename ARCH_TUPLE to LIB_ARCH_TUPLE This is really just about library locations, hence clarify that we don't assume this to be anything but that.

path: add new "systemd-path" utility for querying paths described in file-hierarchy(7) This new tool is based on "sd-path", a new (so far unexported) API for libsystemd, that can hopefully grow into a workable API covering /opt and more one day.