Wait for async command exit

[juergenhoetzel]

This prevents the accumulation of zombie processes when using
async (&) event commands. Also log async command failures.

For example when using

filebrowser cmds add before_copy 'true&'

the following zombie processes remain after 3 copies:

 232983 pts/3    Z+     0:00 [true] <defunct>
 232989 pts/3    Z+     0:00 [true] <defunct>
 233001 pts/3    Z+     0:00 [true] <defunct>

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This commit also logs failures of async commands:

[INFO] Nonblocking Command "false" failed: exit status 1