tftpd: always use curl's own tftp.h

tftpd: always use curl's own tftp.h

Using the system's provided arpa/tftp.h and optimizing, GCC 12 detects
and reports a stringop-overread warning:

tftpd.c: In function ‘write_behind.isra’:
tftpd.c:485:12: warning: ‘write’ reading between 1 and 2147483647 bytes from a region of size 0 [-Wstringop-overread]
  485 |     return write(test->ofile, writebuf, count);
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from tftpd.c:71:
/usr/include/arpa/tftp.h:58:30: note: source object ‘tu_data’ of size 0
   58 |                         char tu_data[0];        /* data or error string */
      |                              ^~~~~~~

This occurs because writebuf points to this field and the latter
cannot be considered as being of dynamic length because it is not
the last field in the structure. Thus it is bound to its declared
size.

This commit always uses curl's own version of tftp.h where the
target field is last in its structure, effectively avoiding the
warning.

As HAVE_ARPA_TFTP_H is not used anymore, cmake/configure checks for
arpa/tftp.h are removed.

Closes #11897