makesyscalls.lua: improve config processing

The current version will strip out #include directives appearing inside strings, which is clearly wrong. Improve the processing entirely in the following ways:

• Strip only whole-line comments on every single iteration
• Abort if we see a malformed line that doesn't match the key=value format
• For quoted (backtick or double quote) strings, we'll advance to the end of the key=value pair and make sure there's not extra stuff left over
• For unquoted key=value pairs, we'll strip any trailing comments and verify there's no internal whitespace

This has revealed the caveat that key/value pairs can't even include escaped quotes (and haven't been able to). I don't know if this is actually problematic, as we're usually looking at cases like "#include <foo>" or raw identifiers.The current version will strip out #include directives appearing inside strings, which is clearly wrong. Improve the processing entirely in the following ways:

Reviewed and noticed by: brooks
Differential Revision: https://reviews.freebsd.org/D22698