factor out internal_split_cookies from auth-juniper.c

factor out internal_split_cookies from auth-juniper.c

This is useful for other protocols that use HTTP cookies for authentication, and may
need a way to handoff >1 cookie from the authentication phase to the connection phase.

Improve it slightly by allowing it to set a "default" HTTP cookie if the
authcookie string doesn't contain '=', since most protocols really only NEED
one cookie for the connection phase to work.

This allows shortcuts (like `openconnect --protocol=nc -C 'foobar'` → 'Cookie: DSID=foobar'
or `openconnect --protocol=fortinet -C 'ABCD123456=='` → 'Cookie: SVPNCOOKIE=ABCD123456==') without limiting
the ability to store multiple cookies if/when useful.

Signed-off-by: Daniel Lenski <dlenski@gmail.com>