tftpd: convert 6 global variables into local ones
tftpd: support "writedelay" within <servercmd>
TFTP: make the CURLOPT_LOW_SPEED* options work

... this also makes sure that the progess callback gets called more
often during TFTP transfers.

Added test 1238 to verify.

Bug: http://curl.haxx.se/bug/view.cgi?id=1269
Reported-by: Jo3