Sync with GNU grep.
Remove Ultrix 4.3 support in custom.h.
Fix real preface for docbook.
Merge branch 'gawk-4.1-stable'
Edits through Chapter 11, fix displays for docbook.
Merge branch 'gawk-4.1-stable'
Edits through chapter 14.
Merge branch 'gawk-4.1-stable'
Edits through Chapter 16.
Merge branch 'gawk-4.1-stable'