lib/util/charset: add back loading of charset modules

lib/util/charset: add back loading of charset modules

For autoconf builds these remain as modules, for waf builds they are
built into the charset library.

This is required to provide the CP850 charset when iconv is not available.

The charset modules static for the waf builds because with proper
shared libs, there isn't the same need for these to be in seperate .so
files.  The modules are also not produced if a system iconv is found,
except for developers, to allow testing of both code paths.

Andrew Bartlett