Make things compile neatly under Sun's C compiler.

Make things compile neatly under Sun's C compiler.
* dynl.c (scm_dynamic_func): Cast return value from sysdep_dynl_func.
* extchrs.c (xmbtowc): Make the second arg a normal char, not
unsigned, because that's what the ANSI function takes.
* extchrs.h (xmbtowc): Corresponding change to prototype.
* genio.c (scm_gen_getc): Make buf plain chars.  Nobody wants
uchars here.
* mbstrings.c (scm_mb_ilength): Use ANSI arg syntax.  Make DATA
argument plain char *.
* strings.c (scm_string): Use SCM_ROCHARS, since c is a plain
char.
* tag.c (scm_tag): Remove unreachable statement.
* unif.c (scm_array_to_list): If we want to shift a 1 bit to the
top of the word, it should be unsigned.