Removed the static m_getfld() state instance and replaced

Removed the static m_getfld() state instance and replaced
with functions for callers to create and destroy their own
instances.  This also allows inc, msh, and pick to call
m_getfld() with an initial file stream position other than 0.

There is now a global state instance so that scan can call
m_unknown() on maildrop files.  It'd be nice to get rid of
that.