The exception handler code still contains a fair amount of useless stuff including:

• The mechanism used to serialize all the crash annotations in a single string
• Unused character-conversion functions
• Overly complicated functions for opening files

This should be cleaned up in preparation for bug 1420363.

This patch includes multiple changes cleaning up various aspects of the crash
reporter client and exception handler:

• Some Unix-specific code was moved out from the base crashreporter client
  code and into the appropriate platform implementation
• Functions used to open files in the crashreporter client now accept C++
  std::ios flags instead of unreadable booleans
• Useless character conversion routines were removed from the
  minidump-analyzer
• Crash annotations are not serialized into a huge string anymore every time
  they change. They are all written out individually during an exception.
• WriteEscapedMozCrashReason() uses the exception-safe my_strlen() instead
  of plain strlen()
• The Windows-specific DLL-blocklist shutdown was removed from the Linux &
  macOS Breakpad callbacks
• The CrashReporterHost, CrashReporterClient and
  CrashReporterMetadataShmem classes now take nsACString references
  instead of nsCString ones since they never modify their contents

https://hg.mozilla.org/mozilla-central/rev/fa8fbc9069ea