hush: fix NOMMU hangs in pseudo_exec_argv. Add forgotted setting
 of signal mask. Reuse same help string in all shells.

function                                             old     new   delta
builtin_exit                                          49      47      -2
pseudo_exec_argv                                     149     145      -4
builtin_help                                          74      63     -11
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-17)             Total: -17 bytes
   text    data     bss     dec     hex filename
 825379     476    7616  833471   cb7bf busybox_old
 825341     476    7616  833433   cb799 busybox_unstripped

diff --git a/shell/ash.c b/shell/ash.c
index 1cd2050..fe8c1be 100644
--- a/shell/ash.c
+++ b/shell/ash.c
@@ -12230,7 +12230,9 @@
 	unsigned col;
 	unsigned i;
 
-	out1fmt("\nBuilt-in commands:\n-------------------\n");
+	out1fmt("\n"
+		"Built-in commands:\n"
+		"------------------\n");
 	for (col = 0, i = 0; i < ARRAY_SIZE(builtintab); i++) {
 		col += out1fmt("%c%s", ((col == 0) ? '\t' : ' '),
 					builtintab[i].name + 1);