ash: fix a bug where redirection fds were not closed afterwards.

ash: fix a bug where redirection fds were not closed afterwards.
 optimize close+fcntl(DUPFD) into dup2. add testsuites.

function                                             old     new   delta
copyfd                                                47      68     +21
argstr                                              1311    1298     -13
popredir                                             148     131     -17
redirect                                            1139    1107     -32
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(add/remove: 0/0 grow/shrink: 1/3 up/down: 21/-62)            Total: -41 bytes