Ensure we use value of FS at the time the record was read

[benhoyt]

I just read this comment in onetrueawk's lib.c, and GoAWK (I think) doesn't do this and probably has this issue.

/*
 * POSIX specifies that fields are supposed to be evaluated as if they were
 * split using the value of FS at the time that the record's value ($0) was
 * read.
 *
 * Since field-splitting is done lazily, we save the current value of FS
 * whenever a new record is read in (implicitly or via getline), or when
 * a new value is assigned to $0.
 */
void savefs(void)