Restructure the grabbing functions into mere wrappers of new open and

Restructure the grabbing functions into mere wrappers of new open and
close functions.  Scattered calls to sc_cnputc() and sc_cngetc() were
broken by turning the semi-reentrant inline context-switching code in
these functions into the grabbing functions.  cncheckc() calls for
panic dumps are the main broken case.  The grabbing functions have
special behaviour (mainly screen switching in sc_cngrab()) which makes
them unsuitable as replacements for the inline code.