MFC r345839, r345894, r345897, r345900-r345901, r345976, r346220, r348602, r348767, r348967, r349047-r349048, r351502, r351623, r352364

r345839: Assert that q can't be NULL. 'empty' is always non-NULL when DIREMPTY
r345894: Restore the ability of checking and fixing next free
r345897: Restore lfcl when LOSTDIR's chain was corrupted and overwritten
r345900: Implement checking of .' and ..' entries of subdirectory.
r345901: Fix build.
r345976: Write string constant differently to improve readability.
r346220: Don't cast result from malloc().
r348602: Don't increment cl after increment.
r348767: preen should work independently with alwaysyes and alwaysno.
r348967: Avoid out of boundary access when checking invalid long filenames.
r349047: Blankspace. No actual code change.
r349048: In ask(): override default option if any of alwaysyes/alwaysno/rdonly is
r351502: Comment boot block checks and perform additional sanity checks:
r351623: Remove unneeded blank line. No functional change.
r352364: Avoid mixing cluster numbers and sector numbers. Makes code more readable.