hush: implement -d DELIM option for 'read'

hush: implement -d DELIM option for 'read'

The POSIX standard only requires the 'read' builtin to handle '-r':
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/read.html

However, Bash introduced the option '-d <DELIM>' to override IFS for
just one invocation, and it is quite useful.

We already support this in ash, let's add it to hush, too.

function                                             old     new   delta
builtin_read                                         263     284     +21
.rodata                                           163587  163589      +2
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/0 up/down: 23/0)               Total: 23 bytes

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>