head,tail: use common suffix struct. simplify help text.

function                                             old     new   delta
head_tail_suffixes                                     -      32     +32
head_main                                            415     406      -9
packed_usage                                       29252   29234     -18
tail_suffixes                                         32       -     -32
head_suffixes                                         32       -     -32
------------------------------------------------------------------------------
(add/remove: 2/2 grow/shrink: 0/2 up/down: 32/-91)            Total: -59 bytes
   text    data     bss     dec     hex filename
 890474     497    7584  898555   db5fb busybox_old
 890415     497    7584  898496   db5c0 busybox_unstripped

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
head: support -n -NUM and -c -NUM

function                                             old     new   delta
head_main                                            406     832    +426
packed_usage                                       29234   29252     +18
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/0 up/down: 444/0)             Total: 444 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
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
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 5/0 up/down: 250/0)             Total: 250 bytes

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