Fixed signed integer overflow errors detected by "clang -fsanitize=undefined",
as reported Alexander Cherepanov
Declared crk_remove_hash() static
Declared itoa64[] static.  Its non-use from outside of common.c was spotted by
Alexander Cherepanov.
Dropped fmt_default_valid().  Its non-use was spotted by Alexander Cherepanov.
Declare rules_errors[], rules_errno, rules_line, and rules_check() static.
Their non-use from outside of rules.c was spotted by Alexander Cherepanov.
Re-introduced exporting of itoa64[] due to its use in jumbo
Moved RULES_ERROR_* and the comments on rules_errors[], rules_errno,
rules_line, and rules_check() from rules.h to rules.c.