Use $PWD as the fallback value for setting initial working directory

This means running "konsole" literally in a terminal will open a konsole
which starts in the same directory as the current terminal. "konsole
--workdir ." is no more needed. This makes Konsole behave like most
other emulators.

Another befinit is it is now easier to use konsole for the "Open
terminal here" feature provided in file managers other than dolphin.
Note: due to the "single process" model of konsole, konsole sometimes
does not open the expected the direcotory when used by this "Open
terminal here" feature.

REVIEW: 104518

FIXED-IN: 4.9.0