tail: make help text for -n +N syntax more correct

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
move endofname() to libbb

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
mdev: add environment variable match

function                                             old     new   delta
make_device                                         1998    2189    +191
clean_up_cur_rule                                     61      96     +35
dirAction                                             75      87     +12
mdev_main                                            838     849     +11
packed_usage                                       29272   29273      +1
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(add/remove: 0/0 grow/shrink: 5/0 up/down: 250/0)             Total: 250 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>