Use sprintf return value instead of invoking strlen on result.

Use sprintf return value instead of invoking strlen on result.

In the old days this wasn't portable, since some sprintf
implementations returned char *.  But they died out years ago and
Emacs already assumes sprintf returns int.
Similarly for float_to_string.
This patch speeds up (number-to-string 1000) by 3% on Fedora 15 x86-64.
* ccl.c (ccl_driver):
* character.c (string_escape_byte8):
* data.c (Fnumber_to_string):
* doprnt.c (doprnt):
* print.c (print_object):
* xdisp.c (message_dolog):
* xfns.c (syms_of_xfns):
Use sprintf or float_to_string result to avoid need to call strlen.
* data.c (Fnumber_to_string):
Use make_unibyte_string, since the string must be ASCII.
* lisp.h, print.c (float_to_string): Now returns int length.
* term.c (produce_glyphless_glyph):
Use sprintf result rather than recomputing it.