* install.md, install-manual.md, and easy-install.md had a lot of
redundancy. Rework them so the common prefix and suffix are in
install.md and it's clear when to navigate back and forth. This
removes from very stale references to prep.sh and cameras.sql in
install-manual.md (which never should have mentioned these scripts
anyway).
* remove all the SAMPLE_MEDIA_DIR, SAMPLE_FILE_DIR, and
SAMPLE_FILE_PATH stuff from the scripts. This was too complicated
(one variable will suffice) and inconsistent in terminology (a
couple "samples dir" occurrences slipped through review; they
should have been "sample file dir"). It also wasn't really useful
enough because the procedure for a mount point is manual anyway,
and because some installs will have multiple sample file dirs
anyway.
* in the mount point procedure, fix the paths to be consistent. Also
describe the "nofail" and "Requires=" config I have on my machine.
* fix some incorrect info about how to use "moonfire-nvr config" and
describe "flush_if_sec".
* Install and build script enhancements
* Use functions for all reporting for consistent output
* Update reporting text to be more clear
* Add some additional messages
* Re-check some install prep items also in install script
* Fix a bug where the wrong db directory would be configured to the service
* Satisfy PR requested changes (spelling mostly).
