Cleanup: nullptr and boolean literals instead of NULL, 0 or 1

[rubidium42]

Motivation / Problem

Inconsistent nullptr usage, or lack thereof in some places where it still used NULL or 0.
Inconsistent boolean usage, where it would use 0 or 1 to denote false or true.

Description

Mostly the result of some experimentation with clang-tidy.
Next to nullptr and boolean literals, also some simplifications of boolean logic, e.g. e?true:false -> e != 0.
Also a duplicate set of lines between both base_station_base.h and station_base.h was removed from the latter, as the latter already includes the former.

Limitations

Have checked only code that was compiled by clang on my computer, so there might be other places.

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[LordAro]

I feel like squirrel was left alone on purpose during the 'main' NULL -> nullptr conversion. Not that it particularly matters, it's so custom at this point that any upstream changes are infeasible anyway...