Development version 3.43d - minor enhancement

(1) Use the "safe print" routine to avoid an additional error while trying
to print a line during exception handling.
Initial implementation of .h4, .h5, .h6 directives
Rework of .h"n" code for code consistency
Merge Develop 3.43e into Master; reset version to 3.44
Merge branch 'develop' into h456
Merge branch 'master' of https://github.com/wf49670/ppgen.git into develop
Merge branch 'develop' into h456
Merge branch 'develop' of https://github.com/wf49670/ppgen.git into h456
3.44b: support for .h4/5/6

Also: ppgen will now warn if the file (fn=) specified on a .il command
does not exist in the images directory. Previously it checked only the
file specified on the link= operand of the .il command.