Attempt to deal with a compile failure that shows up with gcc, which
has -Werror on.  Without this, yylex() is inconsistently or redundantly
defined.
As a followup to r252547, propate const down the call stack.
Make it clear that there are three separate internal locks.
Code cleanups.

MFC after: 3 days
Add descriptions to the FILES list, fix some mdoc complaints, move the
"neither" entry out of the table in BOOTING.

Submitted by:	joeld
Reviewed by:	joeld
MFC after:	1 week
Pull in r185446 from clang trunk:

  Fix to PR15826 - clang hits assert in clang::ASTContext::getASTRecordLayout.

Reported by:	glebius