shell: make it possible to alias one of shells to "bash"

function                                             old     new   delta
packed_usage                                       27047   27054      +7
applet_names                                        2227    2232      +5
applet_main                                         1304    1308      +4
applet_nameofs                                       652     654      +2
applet_install_loc                                   163     164      +1
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 5/0 up/down: 19/0)               Total: 19 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/include/usage.h b/include/usage.h
index 3fce939..a295ab8 100644
--- a/include/usage.h
+++ b/include/usage.h
@@ -123,6 +123,8 @@
 #define lash_full_usage ""
 #define msh_trivial_usage NOUSAGE_STR
 #define msh_full_usage ""
+#define bash_trivial_usage NOUSAGE_STR
+#define bash_full_usage ""
 
 #define awk_trivial_usage \
        "[OPTIONS] [AWK_PROGRAM] [FILE]..."