* src/lread.c (infile): Set/reset it like a dynamically scoped variable

I've seen segfaults where `infile` is nil when we get to
readbyte_from_file, presumably because Fload set it to NULL (via
close_infile_unwind) just before returning to its caller which was
probably itself within another read/load and for some reason
readevalloop didn't get to re-set `infile` like it used to do at every
iteration.  I was not able to really track down the bug, but the way
`infile` was set/reset seemed fragile and managing it like a standard
dynamically-scoped var seems both safer (and more efficient since we
don't need readevalloop to constantly re-set it).

(readchar): Assert that `infile` is set if using a function the depends on it.
(readbyte_from_file): Assert that `infile` is set.
(close_infile_unwind): Reset `infile` to its previous value rather than
to NULL.
(Fload): Remember the previous value of `infile` before chaning it.
(readevalloop): Don't set `infile` any more.

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