Convert inc and scan from m_getfld() to m_getfld2()

The main conversion was done in uip/scansbr.c.

Mbox handling is now completely outside of m_getfld2(), which does not
even know about From_ lines.

No more need to call thisisanmbox() (formerly m_unknown(), ``the magic
invocation'') before calling scan().

The newly introduced gotos should be eliminated ...