Corrected numerous output and string regressions.

reset_call_stack_root() now resets such root only if standard output for
the current shell is attached to a terminal. print_string_sans_newline()
now calls reset_call_stack_root() only if the passed string excluding ANSI
color codes is nonempty. print_string() now always returns zero exit
status (i.e., success). All get_string_length_*() functions now return 0
when passed the empty string.

"And dancing windward transpired
 respiring breath on
 browbeaten lovers' brows,
 perspiring rain song and
 tenderest, unresting mist."