MFC r286368: Remove guards around overwriting loader.rc and menu.rc

There have been .local version of each for user modifications for some time
This allows users to receive future updates to these files

PR: 183765
Differential Revision: https://reviews.freebsd.org/D3176
Submitted by: Bertram Scharpf, Nikolai Lifanov (patch)
Reviewed by: dteske, loos, eadler
Approved by: bapt (mentor)
Relnotes: yes
Sponsored by: ScaleEngine Inc.

MFC r258270:

Add a try-include word (which acts the same as "include") and use it to
conditionally include (but ignore failures) /boot/loader.rc.local and
/boot/menu.rc.local -- to make customizing the menu easier.

Reviewed by: alfred
Discussed on: -hackers

MFC r257650:

Defer loading of kernel and modules if the beastie menu is enabled. Add a
kernel selection menu to the beastie menu. List of kernels is taken from
`kernels' in loader.conf(5) as a space (or comma) separated list of names
to display (up to 9). If not set, default value is "kernel kernel.old".
Does not validate that kernels exist because the next enhancement will be
to allow selection of the root device.

Discussed on: -current

MFC r258270:

Add a try-include word (which acts the same as "include") and use it to
conditionally include (but ignore failures) /boot/loader.rc.local and
/boot/menu.rc.local -- to make customizing the menu easier.

Reviewed by: alfred
Discussed on: -hackers

MFC r257650:

Defer loading of kernel and modules if the beastie menu is enabled. Add a
kernel selection menu to the beastie menu. List of kernels is taken from
`kernels' in loader.conf(5) as a space (or comma) separated list of names
to display (up to 9). If not set, default value is "kernel kernel.old".
Does not validate that kernels exist because the next enhancement will be
to allow selection of the root device.

Discussed on: -current