ApiMutableTableModelControllerBase

(base) Added hooks that can be used by controllers to take actions at the appropriate time during an action

Omit the console changes, we are not prepared for improving
the translation there.

PR: https://forum.opnsense.org/index.php?topic=3502

Changed base class from ApiControllerBase to ApiMutableModelControllerBase
Fixed incorrect string concatenation (+ instead of .) in getNodes()
Removed stray parentheses
Extracted static fields for gridFields and gridDefaultSort

o Recovery console is weird, just use CTRL-C from anywhere
o Simplify the early installer invoke by probing for keypress
o Display a brief message when installer+SSH is active

Prefer LiveCD boot during install media boot, then
set up an extra user for installation.  This user
shall be reached via console or SSH soon enough for
a sort of remote install experience.

PR: https://forum.opnsense.org/index.php?topic=3403

Inspired by: DragonFlyBSD (and we're using its installer anyway!)

This reverts commit 6c6b861e.

(mvc) merge ApiModelControllerBase and ApiMutableModelControllerBase + define class statics properly. as discussed at https://github.com/opnsense/core/pull/1124

PPPoE wasn't removed, which led to some refactoring to find
out why.  The outcome is the same, but works a little bit
different now.  While there, make it possible to omit the
config write in case we are going to call write_config()
shortly after anyway.

This will also be necessary for the legacy VPN plugins.