Deprecate serving files by relative paths.
Deprecate serving files by relative paths.

Relative paths assume that cwd is 'safe'. Instead we should require
people to be more explicit in their paths.

Refs #11921
Refs #11926

tenkoma/cakephp,cakephp/cakephp,garethellis36/cakephp,mvdriel/cakephp,petteyg/cakephp,dakota/cakephp,garethellis36/cakephp,garethellis36/cakephp,mvdriel/cakephp,cakephp/cakephp,ADmad/cakephp,davidyell/cakephp,petteyg/cakephp,dakota/cakephp,ndm2/cakephp,davidyell/cakephp,ADmad/cakephp,garethellis36/cakephp,ndm2/cakephp,ADmad/cakephp,tenkoma/cakephp,CakeDC/cakephp,CakeDC/cakephp,ndm2/cakephp,cakephp/cakephp,dakota/cakephp,dakota/cakephp,tenkoma/cakephp,mvdriel/cakephp,CakeDC/cakephp,CakeDC/cakephp,tenkoma/cakephp,ndm2/cakephp,mvdriel/cakephp,ADmad/cakephp,cakephp/cakephp,petteyg/cakephp,davidyell/cakephp,petteyg/cakephp