maint: simplify partial writes etc. in 'yes'

maint: simplify partial writes etc. in 'yes'

* src/yes.c: Include full-write.h.
(main): Use full_write, not write, to simplify handling of partial
writes.  Don't bother using stdio to output data; just use
full_write with a buffer as large as needed.  Reuse operand
strings if possible, and if the buffer would otherwise be large.