NLS cleanup in ecpglib

Replace leftover instances of _() by ecpg_gettext(), the latter being the
correct way to refer to the library's message catalog, instead of the one of
the program using the library.

Drop NLS support for ecpg_log(), which is a debugging instrument similar to
elog() in the backend.

We cannot support NLS in the ecpg compatlib, because that requires
ecpg_gettext, which is in ecpglib, which is not a dependency of compatlib.  It
doesn't seem worthwhile to worry about this, since the only translatable
string is "out of memory", and gettext probably won't be able to do much
without memory either.

Adjust messages to project style.