process-util: rename get_parent_of_pid() → get_process_ppid() In order to match the other get_process_xyz() calls.

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

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.

basic: rework virtualization detection API Introduce a proper enum, and don't pass around string ids anymore. This simplifies things quite a bit, and makes virtualization detection more similar to architecture detection.

sd-bus: properly handle creds that are known but undefined for a process A number of fields do not apply to all processes, including: there a processes without a controlling tty, without parent process, without service, user services or session. To distuingish these cases from the case where we simply don't have the data, always return ENXIO for them, while returning ENODATA for the case where we really lack the information. Also update the credentials dumping code to show this properly. Fields that are known but do not apply are now shown as "n/a". Note that this also changes some of the calls in process-util.c and cgroup-util.c to return ENXIO for these cases.

shared: add terminal-util.[ch]

shared: add process-util.[ch]