libbb: optionally support RTMIN[+n] and RTMAX[-n] signal names

function                                             old     new   delta
get_signum                                           140     336    +196
__libc_allocate_rtsig                                  -      56     +56
__libc_current_sigrtmin                                -       6      +6
__libc_current_sigrtmax                                -       6      +6
current_rtmin                                          -       4      +4
current_rtmax                                          -       4      +4
bbconfig_config_bz2                                 4961    4962      +1
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(add/remove: 6/0 grow/shrink: 2/0 up/down: 273/0)             Total: 273 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/libbb/Config.src b/libbb/Config.src
index f6c7a11..85892d3 100644
--- a/libbb/Config.src
+++ b/libbb/Config.src
@@ -135,7 +135,7 @@
 	  and create a regular file. This does not conform to POSIX,
 	  but prevents a symlink attack.
 	  Similarly, "cp file device" will not send file's data
-	  to the device.
+	  to the device. (To do that, use "cat file >device")
 
 config FEATURE_VERBOSE_CP_MESSAGE
 	bool "Give more precise messages when copy fails (cp, mv etc)"