xdr: store chars consistently

Cast char's through unsigned char before storing as an integer in
xdr_char(), this ensures that the encoded form is consistently not
sign-extended following Open Solaris's example.

Prior to this change, platforms with signed chars would sign extend
values with the high bit set but ones with unsigned chars would not
so 0xff would be stored as 0x000000ff on unsigned char platforms and
0xffffffff on signed char platforms. Decoding has the same
result for either form so this is a largely cosmetic change, but it
seems best to produce consistent output.

For more discussion, see https://github.com/openzfs/zfs/issues/14173

Reviewed by: mav, imp
Differential Revision: https://reviews.freebsd.org/D37992

(cherry picked from commit a872c37054172f3f7a03aef263ca5886a749771f)