Use default external browser by default on Haiku

Use default external browser by default on Haiku

* lisp/net/browse-url.el (browse-url-default-browser): Use that
by default on Haiku.
(browse-url-default-haiku-browser): New function.
* src/haiku_support.cc (be_roster_launch):
* src/haiku_support.h: New function.  Update prototypes.
* src/haikuselect.c (haiku_message_to_lisp): Encode and decode
files correctly.
(haiku_lisp_to_message): Encode and decode files correctly.
(Fhaiku_roster_launch): New function.
(syms_of_haikuselect): Update defsubrs.