many thanks to @Lompik
* don't match empty unit names. AIUI unit names are alphanumeric with
the exception of the group [-_.@], non ascii are backslash-escaped.
* more strictly match temp file names: .# prefix, [[:hexdigit:]]{16}
suffix. the filenames are a unit name with a unit suffix (systemctl
edit --full) or just override.conf (edit without --full option)
* add regex for dropin config files. This is just a file with ".conf"
extension, with the added constraint that it's a file having some
parent directory named "systemd"
hopefully didn't break everything \o/
* eventual goal is to have the large lists generated at byte compile
time from the text files now present in the repo. updating the lists
in lisp is very annoying
* capf completion now exists and draws from the same infrastructure
* added capability to add a "=" after company completion of a directive
* systemd-use-company-p still works, but does not need to exist
* adds 16 random hex digits to the end of file name
* instead of a bunch of autoloads, draw regexp into a pair of defconst
* not sure how to splice a reusable list of extensions into an rx call,
which would be nicer than duplicating it
