branches: 1.26.40; 1.26.46; 1.26.60;
* When all columns are given null customised headers, the blank header
line is not printed at all. This is specified in P1003.1-2004
(SUSv3), and is useful.

* Customised headers may contain embedded space, commas and equals
signs. To specify multiple customised headers, use multiple -o or -O
options. This is specified (for "-o", not for "-O") in P1003.1-2004
(SUSv3), and is useful.

* When a column is given a null (blank) customised header, it keeps its
default minimum width. This is specified in P1003.1-2004 (SUSv3), and
seems harmless.

* Fix a bug that made it impossible to print the same keyword multiple
times, with different customised headers each time. (Previously, the
last customised header was used for all instances of the keyword.)

* Make the behaviour of "-O" more useful. The first -O option adds
the default keywords only if there have not yet been any formatting
options, and multiple -O options now insert their keywords in adjacent
positions. Now {ps -j -O %cpu} is like {ps -j} with one extra column;
Previously, it would have had all the columns implied by "-j", plus
all the default columns, plus the extra column specified by "-O".

* Convert from home-grown linked lists to SIMPLEQ lists.

Discussed in tech-userlevel.