Fix portability of printing to files returned by open("/dev/stderr").

The BroFile ctor now wraps /dev/std{in,out,err} string arguments
into the actual FILE* provided by stdio.h because use of the former
directly isn't POSIX compliant and led to subtle differences that
broke unit tests on certain platforms (e.g. OS X redirection of stderr
behavior started differing from Linux).  The BroFile (un)serialization
methods already did this kind of logic, so adding it in the ctor also
should make things more consistent.

Some of the reporter-related unit tests looked like they were missing
output because of this, and the coverage test for bare-mode errors
needed tweaking to branch on whether or not libcurl was available
(since the error output differs when elasticsearch isn't there).

testing/btest/Baseline/coverage.bare-mode-errors/unique_errors_no_elasticsearch

@@ -0,0 +1 @@
+error: unknown writer type requested