tree-wide: merge pager_open_if_enabled() to the pager_open()

tree-wide: merge pager_open_if_enabled() to the pager_open()

Many subsystems define own pager_open_if_enabled() function which
checks '--no-pager' command line argument and open pager depends
on its value. All implementations of pager_open_if_enabled() are
the same. Let's merger this function with pager_open() from the
shared/pager.c and remove pager_open_if_enabled() from all subsytems
to prevent code duplication.