List of bugs fixed.

1. ofs-rebuild: OFS value used to rebuild the record was incorrect.
Fixed August 19, 2014. Revised fix August 2018.

2. system-status: Instead of a floating-point division by 256, use
the wait(2) macros to create a reasonable exit status. Fixed March 12, 2016.

3. space: Use provided xisblank() function instead of ispace() for
matching [[:blank:]].

4. a-format: Add POSIX standard %a and %A to supported formats. Check
at runtime that this format is available.

5. decr-NF: Decrementing NF did not change $0. This is a decades-old
bug. There are interactions with the old and new value of OFS as well.
Most of the fix came from the NetBSD awk.

6. string-conv: String conversions of scalars were sticky.  Once a
conversion to string happened, even with OFMT, that value was used until
a new numeric value was assigned, even if OFMT differed from CONVFMT,
and also if CONVFMT changed.

7. unary-plus: Unary plus on a string constant returned the string.
Instead, it should convert the value to numeric and give that value.

X. numeric-subsep: If SUBSEP was set to a numeric value, then its string
value wouldn't always be generated before being needed.

X. subsep-overflow: The length of SUBSEP needs to be rechecked after
calling execute(), in case SUBSEP itself has been changed.