client-conf: Don't create multiple cookie files
The old code loaded cookies at the time of loading the client
configuration, which could lead to creation of multiple cookie files.
For example, when pa_client_conf_load() was called, the default cookie
file was created, and then if PULSE_COOKIE was set,
pa_client_conf_env() would create another cookie file.
This patch moves the loading of the cookie to a separate function,
which pa_context calls just before needing the cookie, so the cookie
won't be loaded from the default file if PULSE_COOKIE is set. This
patch also splits the single cookie and cookie_file fields in
pa_client_conf into multiple fields, one for each possible cookie
source. That change allows falling back to another cookie source if
the primary source doesn't work.
BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=75006