Fix a typo on README.md
Replace use of min(max(...), ...) with clamp(...)
Fix indentation in Dialog.cpp
Only warn once for unknown an compiler / OS
Move OS/distro detection out of ErrorReport.cpp

Also add support for /etc/os-release.
CrashHandlerPOSIX: Don't pre-fork a crash handler

Always starting a process to handle crashes is way too pessimistic
and there are so many other things that may go wrong (we are already
crashing after all), we may as well do an "unsafe" fork().