kmk: Move configure target and buildid generation to relevant makefiles.

Having it in Confkg.kmk is not correct since it makes them part of any make
invocation so that things like kmk clean will clean them too which is
generally wrong.
kmk: Add config-files unit as a more generic version of config_headers.

The config_headers unit will be superceded by it in the next commit.
kmk: Use config-files unit instead of config_headers.

The config-headers unit is more generic, it allows to handle arbitrary
autoconf-style config files. It's also simpler and cleaner than
config_headers (e.g. it gets installed via the normal USES feature).
kmk: Be more quiet.
kmk: Properly define NSPR_LIBS and NSPR_INCS.
kmk: Produce less warnings when compiling.