read-file: reorganize to avoid various issues

read-file: reorganize to avoid various issues

* lib/read-file.c (fread_file): Read 1 more byte than is
currently in a regular file, to immediately detect EOF,
and thus avoid any realloc()s.  As well as being slower,
these may fail, thus artificially limiting the supported size.
Allocate up to SIZE_MAX for streams, rather than limiting
to about SIZE_MAX - SIZE_MAX/5.
Don't use the 'size + BUFSIZ + 1' expression, which
could overflow and cause invalid operation.
As a style decision, explicitly check for overflow rather
than using a temporary roll over variable (new_alloc).