man: accept a list of dirs in $MANPATH

function                                             old     new   delta
add_MANPATH                                            -     143    +143
man_main                                             852     731    -121
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 0/1 up/down: 143/-121)           Total: 22 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Merge remote-tracking branch 'upstream/master' into busybox-next