install: implement -t DIR

install: implement -t DIR

Some packages want to install themselves using "-t" to specify
the directory (as supported by GNU coreutils). Add support for the option
for compatibility reasons.

function                                             old     new   delta
install_longopts                                      76      95     +19
install_main                                         769     777      +8
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(add/remove: 0/0 grow/shrink: 2/0 up/down: 27/0)               Total: 27 bytes

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>