libbb: consolidate the code to set termios unbuffered mode

function                                             old     new   delta
set_termios_to_raw                                     -     116    +116
count_lines                                           72      74      +2
powertop_main                                       1458    1430     -28
top_main                                             943     914     -29
more_main                                            759     714     -45
fsck_minix_main                                     2969    2921     -48
conspy_main                                         1197    1135     -62
rawmode                                               99      36     -63
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 1/6 up/down: 118/-275)         Total: -157 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/include/libbb.h b/include/libbb.h
index abdc8c2..87f89c7 100644
--- a/include/libbb.h
+++ b/include/libbb.h
@@ -1438,6 +1438,10 @@
 int get_terminal_width(int fd) FAST_FUNC;
 
 int tcsetattr_stdin_TCSANOW(const struct termios *tp) FAST_FUNC;
+#define TERMIOS_CLEAR_ISIG (1 << 0)
+#define TERMIOS_RAW_CRNL   (1 << 1)
+#define TERMIOS_RAW_INPUT  (1 << 2)
+int set_termios_to_raw(int fd, struct termios *oldterm, int flags) FAST_FUNC;
 
 /* NB: "unsigned request" is crucial! "int request" will break some arches! */
 int ioctl_or_perror(int fd, unsigned request, void *argp, const char *fmt,...) __attribute__ ((format (printf, 4, 5))) FAST_FUNC;