Libfaketime patch: process-shared "start at"

Dear Wolfgang,

please find a patch that enables "persistent" start-at dates with the "^2010-..." relative syntax. This allows things such as:

   faketime -f '^2002-12-24 20:30:00' /bin/bash -c 'date; sleep 2; date'

as described in he update documentation. The patch extends the existing shm-based mechanism. 

A couple of local changes to make (65df40a) and clock_gettime (4a4673d) were required for my systems, but are otherwise unrelated.

Best