loader.efi environment related cleanups

Since we have dedicated libefi/env.c file for variable support, the following
changes are done:

Simple cstyle changes in env.c
Moved efi variable related commands from loader/main.c to libefi/env.c
Did create function to set "efi-version" environment variable in env.c.

This function does serve two purposes: for first a small clean up of the
loader main(), and for second, it does replace the otherwise unused
efi_variable_support hack.
A bit of cleanup of ficl backend functions. The TEST_MAIN has no meaning,
and removed few memory leaks.

The forth code is updated to use "efi-version" variable, instead of ficl
environment check.

Reviewed by: imp
Approved by: imp (mentor)
Differential Revision: https://reviews.freebsd.org/D9165