Corrected "rsync"-related regressions.

:run_code_with_priorities_nice_and_ionice() now implements safeguards reducing
the likelihood of such regressions erupting again. Also added
:run_code_and_set_string_to_stdout_by_subshell() for performing subshell-based
capturing of standard output, which run_code_paged() in particular now calls.
This should dramatically improve both the efficiency and robustness of paging
under zeshy -- at least, until the next inevitable volly of regressions!