Make pgpmime-support known after Mailvelope init

[pabzm]

The support for pgpmime was only checked initially after the login. If a user e.g. authorizes a domain in Mailvelope only after a login, Mailvelope now instantly works, instead of (silently and undocumentedly) requireing a logout+login-cycle.

[alecpl]

This does not look like a best solution. If you take a look at how mailvelope_load() is invoked you'll see that it is only when Mailvelope is enabled. There's no need to call pgpmime_support_check(). It will not set the env variable in some circumstances.

I see three solutions:

1. I think we should just set rcmail.env.browser_capabilities.pgpmime = 1 in mailvelope_load().
2. Unset $_SESSION['browser_caps']['pgpmime'] when enabling Mailvelope.
3. Always check for pgpmime capability as it is not a heavy check (in browser_capabilities_check()).

2nd is not great as it will require an extra http request. From 1. and 3. I have no preference.

[pabzm]

I chose the first alternative since it's the most straightforward. Thank you for the hint on my misunderstanding of the code!