Recognize 'busybox' in --shell and directives. Add to doc texts.

src/ShellCheck/Checks/ShellSupport.hs

@@ -211,6 +211,8 @@ prop_checkBashisms116 = verify checkBashisms "#!/bin/busybox sh\nx='test'\n${x[1
 prop_checkBashisms117 = verify checkBashisms "#!/bin/busybox sh\nx='test'\n${!x[@]}" -- SC3055
 prop_checkBashisms118 = verify checkBashisms "#!/bin/busybox sh\nxyz=1\n${!x*}" -- SC3056
 prop_checkBashisms119 = verify checkBashisms "#!/bin/busybox sh\nx='test'\n${x^^[t]}" -- SC3059
+prop_checkBashisms120 = verify checkBashisms "#!/bin/sh\n[ x == y ]"
+prop_checkBashisms121 = verifyNot checkBashisms "#!/bin/sh\n# shellcheck shell=busybox\n[ x == y ]"
 checkBashisms = ForShell [Sh, Dash, BusyboxSh] $ \t -> do
     params <- ask
     kludge params t

src/ShellCheck/Data.hs

@@ -156,6 +156,7 @@ shellForExecutable name =
         "sh"    -> return Sh
         "bash"  -> return Bash
         "bats"  -> return Bash
+        "busybox"  -> return BusyboxSh -- Used for directives and --shell=busybox
         "busybox sh"  -> return BusyboxSh
         "busybox ash"  -> return BusyboxSh
         "dash"  -> return Dash