hush: add bash-compatible EPOCH variables

hush: add bash-compatible EPOCH variables

Bash 5.0 added the dynamic variable EPOCHSECONDS and EPOCHREALTIME
which return the number of seconds since the Unix Epoch as an
integer or float.  These are useful for logging or tracing.

function                                             old     new   delta
get_local_var_value                                  207     302     +95
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/0 up/down: 95/0)               Total: 95 bytes
   text	   data	    bss	    dec	    hex	filename
 938702	   4203	   1888	 944793	  e6a99	busybox_old
 938797	   4203	   1888	 944888	  e6af8	busybox_unstripped

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>