utils: Provide a path_absolute() function to check path for non-relativeness

The usually used trivial '/' check won't work on Windows platforms.

src/libstrongswan/utils/utils.c

@@ -288,6 +288,33 @@ char* path_basename(const char *path)
 	return trail ? strndup(pos, trail - pos) : strdup(pos);
 }
 
+/**
+ * Described in header.
+ */
+bool path_absolute(const char *path)
+{
+	if (!path)
+	{
+		return FALSE;
+	}
+#ifdef WIN32
+	if (strpfx(path, "\\\\"))
+	{	/* UNC */
+		return TRUE;
+	}
+	if (strlen(path) && isalpha(path[0]) && path[1] == ':')
+	{	/* drive letter */
+		return TRUE;
+	}
+#else /* !WIN32 */
+	if (path[0] == DIRECTORY_SEPARATOR[0])
+	{
+		return TRUE;
+	}
+#endif
+	return FALSE;
+}
+
 /**
  * Described in header.
  */

src/libstrongswan/utils/utils.h

@@ -568,6 +568,14 @@ char *path_dirname(const char *path);
  */
 char *path_basename(const char *path);
 
+/**
+ * Check if a given path is absolute.
+ *
+ * @param path		path to check
+ * @return			TRUE if absolute, FALSE if relative
+ */
+bool path_absolute(const char *path);
+
 /**
  * Creates a directory and all required parent directories.
  *