tree-wide: use "hostname" spelling everywhere

It's not that I think that "hostname" is vastly superior to "host name". Quite
the opposite — the difference is small, and in some context the two-word version
does fit better. But in the tree, there are ~200 occurrences of the first, and
>1600 of the other, and consistent spelling is more important than any particular
spelling choice.

src/shared/install-printf.c

@@ -115,7 +115,7 @@ int install_full_printf(const UnitFileInstallInfo *i, const char *format, char *
          * %U the UID of the running user
          * %u the username of running user
          * %m the machine ID of the running system
-         * %H the host name of the running system
+         * %H the hostname of the running system
          * %b the boot ID of the running system
          * %v `uname -r` of the running system
          */