Semi-undocumented functions in libpq

Comparing the lists of functions exported by libpq and those declared
by libpq-fe.h turns up a fair number of descrepancies. Most of these
functions are declared by internal header files. For clarity I think we
should clarify the situation, either explicity declare them for
external users, or stop exporting them. Now that we've bumped the major
version of libpq, now is the perfect time to decide.

The functions in question are:

Used by psql:
all the PQExpBuffer functions (./internal/pqexpbuffer.h)
pg_encoding_to_char      (./server/mb/pg_wchar.h)
pqsignal                 (./server/libpq/pqsignal.h)

Used by initdb and createdb:
pg_char_to_encoding      (./server/mb/pg_wchar.h)
Used by initdb:
pg_valid_server_encoding (./server/mb/pg_wchar.h)

Not used by anyone:
pg_utf_mblen             (./server/mb/pg_wchar.h)
pgresStatus              (a way to access info from PQresStatus but not declared publically anywhere)

--
Martijn van Oosterhout   <kleptog@svana.org>   http://svana.org/kleptog/
> From each according to his ability. To each according to his ability to litigate.