journalctl: allow the user to specify the file(s) to use

This is useful for debugging and feels pretty natural. For example
answering the question "is this big .journal file worth keeping?"
is made easier.

src/shared/util.c

@@ -4353,7 +4353,7 @@ int in_group(const char *name) {
 
 int glob_exists(const char *path) {
         _cleanup_globfree_ glob_t g = {};
-        int r, k;
+        int k;
 
         assert(path);
 
@@ -4361,15 +4361,37 @@ int glob_exists(const char *path) {
         k = glob(path, GLOB_NOSORT|GLOB_BRACE, NULL, &g);
 
         if (k == GLOB_NOMATCH)
-                r = 0;
+                return 0;
         else if (k == GLOB_NOSPACE)
-                r = -ENOMEM;
+                return -ENOMEM;
         else if (k == 0)
-                r = !strv_isempty(g.gl_pathv);
+                return !strv_isempty(g.gl_pathv);
         else
-                r = errno ? -errno : -EIO;
+                return errno ? -errno : -EIO;
+}
 
-        return r;
+int glob_extend(char ***strv, const char *path) {
+        _cleanup_globfree_ glob_t g = {};
+        int k;
+        char **p;
+
+        errno = 0;
+        k = glob(optarg, GLOB_NOSORT|GLOB_BRACE, NULL, &g);
+
+        if (k == GLOB_NOMATCH)
+                return -ENOENT;
+        else if (k == GLOB_NOSPACE)
+                return -ENOMEM;
+        else if (k != 0 || strv_isempty(g.gl_pathv))
+                return errno ? -errno : -EIO;
+
+        STRV_FOREACH(p, g.gl_pathv) {
+                k = strv_extend(strv, *p);
+                if (k < 0)
+                        break;
+        }
+
+        return k;
 }
 
 int dirent_ensure_type(DIR *d, struct dirent *de) {

src/shared/util.h

@@ -439,6 +439,7 @@ char* uid_to_name(uid_t uid);
 char* gid_to_name(gid_t gid);
 
 int glob_exists(const char *path);
+int glob_extend(char ***strv, const char *path);
 
 int dirent_ensure_type(DIR *d, struct dirent *de);