MINOR: startup: reintroduce *env keywords support

MINOR: startup: reintroduce *env keywords support

setenv/resetenv/presetenv/unsetenv keywords in the configuration modify the
process environment. In case of master-worker and programs we need to restore
the initial process environment before reload, as the configuration could
change in between and newly forked workers and programs should be launched
in the environment corresponded to this new configuration.

To achieve this we backup the initial process environment before the first
configuration read, when 'global' and 'program' sections are read. And then we
clean up master process environment and restore the initial one from the backup
in mworker_reexec().