tftp: fix bad interaction betweel poll() and alarm(). Closes bug 3061

tftp: fix bad interaction betweel poll() and alarm(). Closes bug 3061

This was breaking timeout handling.

function                                             old     new   delta
tftp_progress_update                                   -      45     +45
tftp_progress_done                                     -      32     +32
tftp_protocol                                       1839    1858     +19
tftp_progress_init                                     9      15      +6
tftp_main                                            298     286     -12
------------------------------------------------------------------------------
(add/remove: 2/0 grow/shrink: 2/1 up/down: 102/-12)            Total: 90 bytes

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>