irc: improve help of server option "autojoin"
core: remove warning about proxy not found for option weechat.network.proxy_curl on startup

Proxies are not yet fully loaded (when reading weechat.conf), so the warning
is not displayed on startup, but only when option is changed.
core: add options to customize default text search in buffers

New options:
- weechat.look.buffer_search_case_sensitive (boolean, off by default)
- weechat.look.buffer_search_force_default (boolean, off by default)
- weechat.look.buffer_search_regex (boolean, off by default)
- weechat.look.buffer_search_where (prefix, message (by default), prefix_message)