* fports.h, fports.c (scm_i_fdes_to_port): New, like

* fports.h, fports.c (scm_i_fdes_to_port): New, like
scm_fdes_to_port, but take mode bits directly instead of as a C
string.
(scm_i_fdes_to_port): Implement using above.
(scm_open_file): Use scm_i_fdes_to_port together with
scm_i_mode_bits to avoid accessing internals of SCM string from C.
* vports.c (scm_make_soft_port): Use scm_i_fdes_to_port together
with scm_i_mode_bits to avoid accessing internals of SCM string
from C.

* ports.h (scm_i_mode_bits): New, same as scm_mode_bits but with a
SCM string as argument.

* ports.c (scm_i_void_port): New, like scm_void_port but take mode
bits directly instead of C string.
(scm_void_port): Implement using above.
(scm_sys_make_void_port): Use scm_i_void_port together with
scm_i_mode_bits to avoid accessing internals of SCM string.

* convert.i.c, backtrace.c, strop.c, strorder.c, strports.c,
struct.c, unif.c, ports.c: Use SCM_I_STRING_CHARS,
SCM_I_STRING_UCHARS, and SCM_I_STRING_LENGTH instead of
SCM_STRING_CHARS, SCM_STRING_UCHARS, and SCM_STRING_LENGTH,
respectively.  Also, replaced scm_return_first with more explicit
scm_remember_upto_here_1, etc, or introduced them in the first
place.