Fix trjconv -sub with .trr files

Fix trjconv -sub with .trr files

ftp is set from the -o file's extension, which cannot be efTRN,
since that's a place-holder file type for command-line parsing.
The error text clearly indicates that .trr (== efTRR) should
work, but of course we have no tests for anything.

Added comment that other trajectory types could work here,
but can't be tested at this time.

Fixes #1118

Change-Id: I118b9b1c25fa7c0640c27e584400dfc64853e67f