shells: remove few statics and duplicated code

shells: remove few statics and duplicated code
(much more of the same remains, alas)
function                                             old     new   delta
doset                                                330     332      +2
warn                                                  53      51      -2
onecommand                                           463     461      -2
ioecho                                                40      38      -2
forkexec                                            1412    1410      -2
err                                                   81      79      -2
setdash                                               59      56      -3
flag                                                   4       -      -4
msh_main                                            1389    1384      -5
eval                                                 388     381      -7
subgetc                                              759     747     -12
static.local                                          14       -     -14
b_adduint                                             70      52     -18
------------------------------------------------------------------------------
(add/remove: 0/2 grow/shrink: 1/10 up/down: 2/-73)            Total: -71 bytes