return of soup_uri_to_string must be free

thank to @fanglingsu
autocmd: add a service function `autocmd_in_use`

The function should permit to check if an AuEvent is actually in use or
not. It would permit to change program behaviour at runtime, and reduce
the overhead of some functions when AuEvent isn't used.
use FEATURE_AUTOCMD for AU_REQUEST_QUEUED

call to autocmd_run need to be conditionnal defined with FEATURE_AUTOCMD
connect `request-queued` signal conditionnally

- add a new function: vb_update_signals()
  it manage signal connection/disconnection

  currently, only `request-queued` is managed. It is connected when
  autocmd use event AU_REQUEST_QUEUED or when setting
  "content-security-policy" is in use.

- remove the unconditionnally connection of `request-queued`

- add a new flag in setting `FLAG_SIGNAL` that inform setting_set_value
  to refresh signals by calling vb_update_signals()

- manage "content-security-policy" setting with `FLAG_SIGNAL`

- refresh signals in autocmd_add, at adding or removing. This permit to
  change the connection/disconnection state of `request-queued` when
  AU_REQUEST_QUEUED is in use or not.
content-security-policy: update documentation

mention RequestQueued as prefered autocmd event for
content-security-policy.