curl: Try to initialize with SSL support to handle https:// URIs

If initialization fails, we fall back to the old behavior.
leak-detective: Whitelist libssl SSL_COMP_get_compression_methods()

This function is called by libcurl initialization with SSL, and uses
a static allocation of compression algorithms not freed.
curl: Dynamically query supported protocols and register appropriate features
configure: Load fetcher plugins after crypto base plugins

Some fetcher plugins (such as curl) might build upon OpenSSL to implement
HTTPS fetching. As we set (and can't unset) threading callbacks in our
openssl plugin, we must ensure that OpenSSL functions don't get called after
openssl plugin unloading.

We achieve that by loading curl and all other fetcher plugins after the base
crypto plugins, including openssl.
curl: For SSL features, depend on thread-safety provided by our crypto plugins

To use SSL in curl, we need to initialize the SSL library in a thread-safe
manner and provide the appropriate callbacks. As we already do that in our
crypto plugins using these libraries, we depend on these features.

This implies that we need the same plugin enabled (openssl, gcrypt) as the
curl backend is configured to use to fetch from HTTPS URIs.
travis: Disable soup in "all" test

On Ubuntu 12.04, there seems to be a resource leak related to pthread keys
when initializing glib or related libraries more than once. With our repeated
initialization for libstrongswan tests, we hit the following error:

  Lib (gthread-posix.c): Unexpected error from C library during
  'pthread_key_create': Resource temporarily unavailable.

The problem is not reproducible on a newer Gnome stack, hence we disable the
glib based soup plugin until we have a more recent Ubuntu on Travis.
Merge branch 'curl-features'

Enable missing https:// support in the curl plugin by initializing libcurl
appropriately.

To initialize the SSL backend properly as required, we rely on our specific
crypto backends (openssl, gcrypt) that already provide this functionality.

Fixes #692.