* utils/hbmk2/hbmk2.prg
* remove `_HBSHELL_EXEC_PRE` macro
+ add `_HBSHELL_EXTRA_HEADER` macro that can be defined to a header
(.ch) filename which will be included in all dot prompt and script
execution sessions. The header will also be embedded in
hbmk2/hbrun at build-time, so it's not necessary to be present at
runtime. The macro may also be defined in the `hbmk2_extra.prg`
source.