action: do not expand env vars in Exec action

...<command> argument (but still resolve tilde).

This makes it easier to write sh -c '' constructs without turning labwc
into a shell parser in order to expand environment variables, whilst
respecting single quotes and escaped characters as well as ignoring
subshells syntax like $(foo) and backticks.

Also, fix bug where buffer length+alloc get out-of-sync

src/config/session.c

@@ -38,6 +38,7 @@ process_line(char *line)
 	buf_init(&value);
 	buf_add(&value, string_strip(++p));
 	buf_expand_shell_variables(&value);
+	buf_expand_tilde(&value);
 	if (string_empty(key) || !value.len) {
 		goto error;
 	}