stand: lines with comments a '"' in loader.conf are ignored

So the problem is that the lua parser in /boot/lua/config.lua is
splitting the line on the '=', sending the first match to the key and
the second to the value.

In the problem case, the value is:

'"test b"               # This is "test_directive_b"'

Then the value gets matched against QVALEXPR = '"(.*)"'.

This cleans up the value by removing the first and last '"', but in this
case turns it into:

test b"               # This is "test_directive_b

which then gets rejected in processEnvVar() with MSG_FAILSYN_QUOTE,
since values aren't allowed to contain '"'.

Changing QVALEXPR to '([-%w_]+)' fixes this problem. Since the value is
explicitly prevented from containing quotes, it's not unreasonable to
load it from an expression that excludes them. The only other place
this is used is in the 'exec="command"' expression, which also should
not contain '"' in the command value.

PR: 265001
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D35975