MFC r300792,r300851,r301580:

r300792:
exec: Add credential change information into imgp for process_exec hook.
r300851:
exec: get rid of one vnode lock/unlock pair in do_execve
r301580:
Old process credentials for setuid execve must not be dereferenced when the
process credentials were not changed. This can happen if an error occured
trying to activate the setuid binary. And on error, if new credentials
were not yet assigned, they must be freed to not create the leak.

MFC r282708:
On exec, single-threading must be enforced before arguments space is
allocated from exec_map.

MFC r281548:
Implement support for binary to request specific stack size for the
initial thread.

MFC r271141: Allow multiple image activators to run on the same
execution by changing imgp->interpreted to a bitmask instead of,
functionally, a bool.

Approved by: re (gjb)

MFC r282708:
On exec, single-threading must be enforced before arguments space is
allocated from exec_map.

MFC r281548:
Implement support for binary to request specific stack size for the
initial thread.

MFC r271141: Allow multiple image activators to run on the same
execution by changing imgp->interpreted to a bitmask instead of,
functionally, a bool.

Approved by: re (gjb)