slirp: Use strcasecmp() to check tftp mode, tsize

According to RFC 1350 (TFTP Revision 2) the mode field can contain any
combination of upper and lower case; also RFC 2349 propagates that the
transfer size option ("tsize") is case in-sensitive too.

Current implementation of embedded TFTP server missed that what does
mess some TFTP clients. Fixed by using STRCASECMP(3) in the required
places.

Signed-off-by: Sergei Gavrikov <sergei.gavrikov@gmail.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Edgar E. Iglesias <edgar@axis.com>