sysusers: add a generic specifier table for common cases

This moves the definition of the specifier table consisting only of
system and /tmp specifiers into generic code so that we can share it.
This patch only adds one user of it for now. Follow-up patches will add
more.

src/shared/specifier.c

@@ -364,3 +364,9 @@ int specifier_escape_strv(char **l, char ***ret) {
 
         return 0;
 }
+
+const Specifier system_and_tmp_specifier_table[] = {
+        COMMON_SYSTEM_SPECIFIERS,
+        COMMON_TMP_SPECIFIERS,
+        {}
+};

src/shared/specifier.h

@@ -80,7 +80,6 @@ int specifier_var_tmp_dir(char specifier, const void *data, const void *userdata
         { 'w', specifier_os_version_id,   NULL }, \
         { 'W', specifier_os_variant_id,   NULL }
 
-
 #define COMMON_CREDS_SPECIFIERS                   \
         { 'g', specifier_group_name,      NULL }, \
         { 'G', specifier_group_id,        NULL }, \
@@ -96,3 +95,6 @@ static inline char* specifier_escape(const char *string) {
 }
 
 int specifier_escape_strv(char **l, char ***ret);
+
+/* A generic specifier table consisting of COMMON_SYSTEM_SPECIFIERS and COMMON_TMP_SPECIFIERS */
+extern const Specifier system_and_tmp_specifier_table[];