Add a talloc context parameter to current_timestring() to fix memleaks.

Add a talloc context parameter to current_timestring() to fix memleaks.

current_timestring used to return a string talloced to talloc_tos().
When called by DEBUG from a TALLOC_FREE, this produced messages
"no talloc stackframe around, leaking memory". For example when
used from net conf.

This also adds a temporary talloc context to alloc_sub_basic().
For this purpose, the exit strategy is slightly altered: a common
exit point is used for success and failure.

Michael