ADDRENV: Use a group flag to determine if there is an address environment (instead of the thread type)

Add group_addrenv() which will be called during context switches in order to change address environments.  Not yet hooked in

Add group_addrenv() which will be called during context switches in order to change address environments.  Not yet hooked in

Rename up_addrenv_assign() to up_addrenv_clone() and generalize its arguments so that can be used for other purposes

Fix confusion about what is a page of data and what is a page of L2 page table; restructure functions to reduce duplicated logic

Change when address environment is instantiated; there are/were locations where access is made to the allocation address environment when it is not yet in place

addrenv interface changes: up_addrenv_create() may need to create .text and .bss/.data separately because of differing access privileges (read/execute vs read/write).  And, as a consequence, up_addrenv_vaddr() needs to be split into up_addrenv_vtext(0 and up_addrenv_vdata().