m_getfld() et al: Add wrapper without FILE *iob parameter.

m_getfld() et al: Add wrapper without FILE *iob parameter.

m_getfld(), m_getfld_track_filepos(), and m_unknown() all take a FILE
*iob that's used to update *m_getfld_state_t, even though it's probably,
hopefully, the same as the previous calls.  Create wrapper functions for
these with an arbitrary `2' suffix that don't have this parameter.
After checking the m_getfld_state_t is initialised, they pass in its
existing FILE *iob.  This allows callers to transition to the wrappers
in the cases where it's obviously the same FILE *iob being passed in.
It isn't always obvious, in part because of the convention for calling
this variable `in', even when it's a file-level static, a word that also
occurs in comments.