Have broctl return a useful exit code

Applied patch from Vlad Grigorescu.

Addresses #920.
Previously-applied patch needs a small adjustment
Previously-applied patch needs another small fix

Patch applied in commit 4274c596d7429c299467130d8816ea97aebe25bc needed
another small fix.
Minor bug fix of the broctl process command

Added a missing "return" statement to prevent trying to run Bro when
there is a syntax error with the broctl "process" command.

Also removed some redundant lines that were setting a non-zero exit code
twice.
Fix broctl exit status for capstats command

Fixed the broctl capstats command to report a non-zero exit status
when an error is encountered.
Previously-applied patch needs another small fix

Patch applied in commit 4274c596d7429c299467130d8816ea97aebe25bc needed
another small fix (a variable that is always a boolean was being checked
for equality to a string).
Add error reporting to more broctl commands

Added error reporting (via a non-zero return code) to the cleanup,
df, install, and print commands.  Also removed some redundant lines
of code in restart command that were setting error code more than once.
Merge remote-tracking branch 'origin/master' into topic/dnthayer/bug920
Merge remote-tracking branch 'origin/master' into topic/dnthayer/bug920
Merge remote-tracking branch 'origin/topic/dnthayer/bug920'

BIT-920 #merged

* origin/topic/dnthayer/bug920:
  Add error reporting to more broctl commands
  Previously-applied patch needs another small fix
  Fix broctl exit status for capstats command
  Minor bug fix of the broctl process command
  Previously-applied patch needs another small fix
  Previously-applied patch needs a small adjustment
  Have broctl return a useful exit code