specifiers: introduce common macros for generating specifier tables

In many cases the tables are largely the same, hence define a common set
of macros to generate the common parts.

This adds in a couple of missing specifiers here and there, so is more
thant just refactoring: it actually fixes accidental omissions.

Note that some entries that look like they could be unified under these
macros can't really be unified, since they are slightly different. For
example in the DNSSD service logic we want to use the DNSSD hostname for
%H rather than the unmodified kernel one.

src/sysusers/sysusers.c

@@ -1391,18 +1391,8 @@ static bool item_equal(Item *a, Item *b) {
 static int parse_line(const char *fname, unsigned line, const char *buffer) {
 
         static const Specifier specifier_table[] = {
-                { 'm', specifier_machine_id,      NULL },
-                { 'b', specifier_boot_id,         NULL },
-                { 'H', specifier_host_name,       NULL },
-                { 'l', specifier_short_host_name, NULL },
-                { 'v', specifier_kernel_release,  NULL },
-                { 'a', specifier_architecture,    NULL },
-                { 'o', specifier_os_id,           NULL },
-                { 'w', specifier_os_version_id,   NULL },
-                { 'B', specifier_os_build_id,     NULL },
-                { 'W', specifier_os_variant_id,   NULL },
-                { 'T', specifier_tmp_dir,         NULL },
-                { 'V', specifier_var_tmp_dir,     NULL },
+                COMMON_SYSTEM_SPECIFIERS,
+                COMMON_TMP_SPECIFIERS,
                 {}
         };