libsa: make single bit bitfields unsigned to avoid clang 16 warning
Clang 16 introduced a warning about single bit bitfields in structs,
which is triggered by a declaration in libsa's tftp.c:
stand/libsa/tftp.c:382:20: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
h->islastblock = 1; /* very short file */
^ ~
stand/libsa/tftp.c:432:18: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
h->islastblock = 1; /* EOF */
^ ~Signed one-bit bitfields can only have values -1 and 0, but the intent
here is to use the field as a boolean, so make it unsigned.
MFC after: 3 days