src/extend.c,src/mainfrm.cc,src/sorterr.c: Fix message references
which I failed to update.
lib/survex.pot: Fix TRANSLATORS comment punctuation to match source
code.
src/labelinfo.h,src/mainfrm.cc: Factor out code to show station name
or "anonymous station".
lib/survex.pot: Update source file references.
lib/: Merge changes from survex.pot to *.po.
lib/en_US.po: Translate new message.