Update the debugging section to include lldb as well.

Clean up the sections a bit to distinguish the two available debuggers (gdb
and llvm). This includes giving the existing gdb sections a title that
ends in "with gdb". The existing gdb section were grouped together into a
new "Using gdb" section and moved up one section level (sect3 to sect2).

A new section "Introduction to available debuggers" introduces and
distinguishes gbd and lldb. Insert a new "Using lldb" section after the
introduction, using the same structure and example code as the gdb section
to ease comparison between the two compilers.

An earlier reference to gdb is replaced by a generic debugger reference.
Include the neccessary whitespace changes. On top of that, update copyright
years of the developers-handbook.

Thanks to Pau Amma for his diligent work on the patch and incorporating
our feedback from the review!

Submitted by: pauamma_gundo.com
Reviewed by: debdrup, woodsb02, emaste, bcr
Approved by: bcr
Differential Revision: https://reviews.freebsd.org/D25139