HEADS-UP: Userland debug files enabled by default

As of r291955 userland debug files are built and installed by default,
in order to facilitate debugging. They will be built as part of the
release process (in FreeBSD 11) so that they can be made available for
download either at install time, or later on to debug a core file
after a crash. (Release builds currently require the use of all
default options.)

The debug files will be located automatically by gdb or lldb, by
following the ".gnu_debuglink" section in the binary or library.

These files occupy additional disk space in the build object directory
(e.g. /usr/obj) and in the install target filesystem (in
/usr/lib/debug/...). If you do not want to build and install the debug
files for any reason, add the following to /etc/src.conf:
WITHOUT_DEBUG_FILES=YES

I hope to refine the option further to provide separate control over
building debug files for binaries and for libraries/rltd.