debug: add new envvar

debug: add new envvar

Add support for 2 new envvars for internal lvm2 test suite
(though it could be possible usable for other cases)

LVM_LOG_FILE_EPOCH

Whether to add 'epoch' extension that consist from
the envvar 'string' + pid + starttime in kernel units
obtained from /proc/self/stat.

LVM_LOG_FILE_UNLINK_STATUS

Whether to unlink the log depending on return status value,
so if the command is successful the log is automatically
deleted.

API is still for now experimental to catch various issue.