core: add comment with return value of function string_strcasestr
core: check that string is not NULL in function string_convert_escaped_chars
core: check that "chars" argument is not NULL in function string_strip
core: add note about result that must be freed in function string_mask_to_regex
core: check that regex is not NULL in function string_regex_flags
core: check that regex is not NULL in function string_regcomp
core: check that timeval arguments are not NULL in timeval functions
core: return only -1, 0 or 1 in string comparison functions
doc: fix example for function strcasecmp_range in Italian plugin API reference