lib-storage: add mail_user_plugin_getenv_bool

New API will check boolean-like env setting OR its exsitence.

Function returns FALSE when:
* Read out a null string,
"Or" * String starts with any letter in set {'N', 'n', 'F','f','0'}

To keep downward compatibility, For other cases including an empty
string, function returns TRUE.

src/lib-storage/mail-user.c

@@ -417,6 +417,25 @@ bool mail_user_is_plugin_loaded(struct mail_user *user, struct module *module)
 	return ret;
 }
 
+bool mail_user_plugin_getenv_bool(struct mail_user *user, const char *name)
+{
+	const char *env = mail_user_set_plugin_getenv(user->set, name);
+
+	if (env == NULL)
+		return FALSE;
+	switch (env[0]) {
+		case 'n':
+		case 'N':
+		case '0':
+		case 'f':
+		case 'F':
+		return FALSE;
+	}
+
+	//any other value including empty string will be treated as TRUE.
+	return TRUE;
+}
+
 const char *mail_user_plugin_getenv(struct mail_user *user, const char *name)
 {
 	return mail_user_set_plugin_getenv(user->set, name);

src/lib-storage/mail-user.h

@@ -146,6 +146,7 @@ void mail_user_set_get_temp_prefix(string_t *dest,
 bool mail_user_is_plugin_loaded(struct mail_user *user, struct module *module);
 /* If name exists in plugin_envs, return its value. */
 const char *mail_user_plugin_getenv(struct mail_user *user, const char *name);
+bool mail_user_plugin_getenv_bool(struct mail_user *user, const char *name);
 const char *mail_user_set_plugin_getenv(const struct mail_user_settings *set,
 					const char *name);