libgcobol: Fix uses of tolower and toupper with std::transform

libgcobol: Fix uses of tolower and toupper with std::transform

As explained in the libstdc++ manual[1] and elsewhere[2], using tolower
and toupper in std::transform directly is wrong. If char is signed then
non-ASCII characters with negative values lead to undefined behaviour.
Also, tolower and toupper are overloaded names in C++ so expecting them
to resolve to a unique function pointer is unreliable. Finally, the
<cctype> header was included, not <ctype.h>, so they should have been
qualified as std::tolower and std::toupper.

[1] https://gcc.gnu.org/onlinedocs/libstdc++/manual/strings.html#strings.string.simple
[2] https://en.cppreference.com/w/cpp/string/byte/tolower

libgcobol/ChangeLog:

	* intrinsic.cc (is_zulu_format): Qualify toupper and cast
	argument to unsigned char.
	(fill_cobol_tm): Likewise.
	(iscasematch): Likewise for to lower.
	(numval): Qualify calls to tolower.
	(__gg__lower_case): Use lambda expression for
	tolower call.
	(__gg__upper_case): Likewise for toupper call.
	* libgcobol.cc (mangler_core): Cast tolower argument to unsigned
	char.
	* valconv.cc (__gg__string_to_numeric_edited): Cast to upper
	arguments to unsigned char.