box: add user_def_new and user_def_delete helpers

We will need to store some extra data in the user_def struct to support
different authentication mechanisms. Let's introduce convenient helpers
for allocating and freeing this struct so that we don't have to patch
all the places in the code where it's allocated or freed when we extend
the struct.

While we are at it, switch to grp_alloc, shorted the license text, and
replace include guards with pragma.

Needed for #7986

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring