lualoader: improve loader.conf var processing

lualoader: improve loader.conf var processing

lualoader was previously not processing \ as escapes; this commit fixes
that and does better error checking on the value as well.

Additionally, loader.conf had some odd restrictions on values that make
little sense. Previously, lines like:

kernel=foo

Would simply be discarded with a malformed line complaint you might not
see unless you disable beastie.

lualoader tries to process these as well as it can and manipulates the
environment, while forthloader did minimal processing and constructed a
`set` command to do the heavy lifting instead. The lua approach was
re-envisioned from building a `set` command so that we can appropriately
reset the environment when, for example, boot environments change.

Lift the previous restrictions to allow unquoted values on the right hand
side of an expression.  Note that an unquoted value is effectively:

[A-Za-z0-9-][A-Za-z0-9-_.]*

This commit also stops trying to weirdly limit what it can handle in a
quoted value. Previously it only allowed spaces, alphanumeric, and
punctuation, which is kind of weird. Change it here to grab as much as it
can between two sets of quotes, then let processEnvVar() do the needful and
complain if it finds something malformed looking.

My extremely sophisticated test suite is as follows:

<<EOF
X_01_simple_string="simple"
X_02_escaped_string="s\imple"

X_03_unquoted_val=3
X_04_unquoted_strval=simple_test

X_05_subval="${X_03_unquoted_val}"
X_06_escaped_subval="\${X_03_unquoted_val}"

X_07_embedded="truth${X_03_unquoted_val}"
X_08_escaped_embedded="truth\${X_03_unquoted_val}"

X_09_unknown="${unknown_val}"
X_10_unknown_embedded="truth${unknown_val}"

X_11_crunchy="crunch$unknown_val crunch"
X_12_crunchy="crunch${unknown_val}crunch"

Y_01_badquote="te"lol"
Y_02_eolesc="lol\"
Y_02_noteolesc="lol\\"
Y_03_eolvar="lol$"
Y_03_noteolvar="lol\$"
Y_04_badvar="lol${"

exec="echo Done!"
EOF

Future work may provide a stub loader module in userland so that we can
formally test the loader scripts rather than sketchy setups like the above
in conjunction with the lua-* tools in ^/tools/boot.

(cherry picked from commit 576562856efbec31520aca6a1f72f2b11298e9a7)