Problem: When in Ex mode and an error is caught by try-catch, Vim still
exits with a non-zero exit code.
Solution: Don't set ex_exitval when inside a try-catch. (partly by Christian
Brabandt)
Problem: To pass buffer content to system() and systemlist() one has to
first create a string or list.
Solution: Allow passing a buffer number. (LemonBoy, closes#1240)
