docs: point out git send-email location, be more stern about make check

docs: point out git send-email location, be more stern about make check

An email came to libvir-list wondering why the git send-email command
was missing in spite of having git installed; this is due to the
send-email command being in a sub-package of the main git package.

While touching the hacking file, I thought it would be useful to 1)
indicate the location of the source (docs/hacking.html.in) in the
message at the top of HACKING, and also to make the note about running
"make check" and "make syntax-check" a bit more stern.