Warning fixes from Maciej Stachowiak:

Warning fixes from Maciej Stachowiak:
* backtrace.h (scm_display_application, scm_backtrace): Add
prototypes.
* debug.c (scm_m_start_stack): Make this function static.
* fluids.h (scm_fluid_p): Add prototype.
* procprop.c (scm_i_procedure_arity): Make this function static.
* regex-posix.c (scm_regexp_error_msg): Make this function static.
* regex-posix.h (scm_init_regex_posix): Use prototype, not K&R decl.
* root.h (scm_dynamic_root): Add external prototype.
* scmsigs.h (scm_usleep): Add external prototype.
* script.h (scm_init_script): Use prototype, not K&R decl.
* stacks.h (scm_stack_id): Add external prototype.
* symbols.h (scm_sysintern0_no_module_lookup): Add external prototype.