edify: Move State.script and State.errmsg to std::string.

This way we kill a few strdup() and free() calls.

Test: 1. recovery_component_test still passes;
2. Applying an update with the new updater works;
3. The error code in a script with abort("E310: xyz") is recorded into
last_install correctly.

Change-Id: Ibda4da5937346e058a0d7cc81764d6f02920010a