BUG/MINOR: mworker: unset more internal variables from program section

BUG/MINOR: mworker: unset more internal variables from program section

People who use HAProxy as a process 1 in containers sometimes starts
other things from the program section. This is still not recommend as
the master process has minimal features regarding process management.

Environment variables are still inherited, even internal ones.

Since 2.7, it could provoke a crash when inheriting the
HAPROXY_STARTUPLOGS_FD variable.

Note: for future releases it should be better to clean the env and sets
a list of variable to be exported. We need to determine which variables
are used by users before.

Must be backported in 2.7.