Implement font-use-system-font on Haiku

Implement font-use-system-font on Haiku

* doc/emacs/frames.texi (Fonts): Update documentation to say
what font-use-system-font really does and where it can be used.
* src/haiku_font_support.cc (language_code_points): Fix coding
style.
(font_style_to_flags, be_font_style_to_flags): Accept const char
*.
(be_send_font_settings, be_listen_font_settings)
(be_lock_font_defaults, be_unlock_font_defaults)
(be_get_font_default, be_get_font_size): New functions used to
retrieve default font data.

* src/haiku_io.c (haiku_len): Handle FONT_CHANGE_EVENT.
* src/haiku_support.h (enum haiku_event_type): New event type
FONT_CHANGE_EVENT.
(enum haiku_what_font): New enum.
(struct haiku_font_change_event): New struct.

* src/haikufont.c (Ffont_get_system_normal_font)
(Ffont_get_system_font, haiku_handle_font_change_event): New
functions.
(syms_of_haikufont): Provide `dynamic-setting' and define new
variables and subrs.

* src/haikuterm.c (haiku_default_font_parameter): Use system font.
(haiku_read_socket): Handle FONT_CHANGE_EVENTS.
(haiku_term_init): Start listening for font configuration changes.
* src/haikuterm.h: Update prototypes.

* src/xsettings.c (Ffont_get_system_normal_font)
(Ffont_get_system_font): Update doc string.
(syms_of_xsettings): Replace calls to intern with a static
string.