fix getenv_s call

- call with first and second argument NULL to check whether var is set is
  invalid and leads to abort on windows

style changes to accomodate update to astyle 3.6.10

update changelog and comments

change loadLibrary on Windows to LoadLibraryA + LoadLibraryExA

- try LoadLibraryA first in any case
- try LoadLibraryExA if that failed
- print error from both, unless second failed with invalid parameter error
- see also #759 and #760

set release date

set release date

set 3.14.13 release date

changelog entry and release date

update option docu

fix that grad_f_cache was not invalidated

- ... when warmstart with same structure and grad_f_constant is set
- need to get new gradient value still, as coefs may have changed for new
  solve