tools: allow tools to use C99

The previous requirement was that tools should only use C89 features.
Several tools did not follow this rule for years. Since nobody
complained about not being able to

tools: allow tools to use C99

The previous requirement was that tools should only use C89 features.
Several tools did not follow this rule for years. Since nobody
complained about not being able to build NetBSD in a C89 environment,
drop that rule, now requiring C99.

https://mail-index.netbsd.org/tech-toolchain/2022/01/30/msg004091.html

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Update documentation about tools being restricted to C89 (or not).

It's not practical for the C89 restriction to be maintained for
compilers, but it is still desirable for most tools. The "long lon

Update documentation about tools being restricted to C89 (or not).

It's not practical for the C89 restriction to be maintained for
compilers, but it is still desirable for most tools. The "long long"
data type is in wide use despite not being in C89. C99 library features
(as opposed to language features) can often be added to the compat
framework when the need becomes apparent.

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Say that tools should use C89, not C99; Say that tools may use
HAVE_NBTOOL_CONFIG_H to conditionally exclude features. Many
other small changes.

It is usually easy to add new definitions to src/tools/compat, and that
is usually better than adding compatibility definitions to individual
tools.

Add tools/README, to collect hints like how to use

.if defined(HOSTPROG)

and

#if HAVE_NBTOOL_CONFIG_H
#include "nbtool_config.h"
#endif /* HAVE_NBTOOL_CONFIG_H */