www/caddy{,-custom}: Fix start/stop with admin API disabled

Caddy largely relies on the admin API for state control, like
start/stop/reload. However, the admin API endpoints are inherently
a security risk.

Although the admin API is enabled by default, many users may choose
to disable it. However, the rc(8) script then needs an alternative
approach to controlling the daemon.

We already sortof supported signal-based control, but it didn't always
work, because there are multiple error messages that indicate that
the admin API is disabled (and none of them actually say that in a clear
way).

This commit fixes start and stop with the admin API disabled. The
reload command (and reloadssl) still require the admin API to be
enabled and will fail if the admin API is disabled.

PR: 255106
Submitted by: scf