Improve POP code, move most of it into a separate file.

Improve POP code, move most of it into a separate file.
(mbx_delimit_end, mbx_delimit_begin): Check for errors.
(mbx_write): Check for errors and for From line.
(pop_retr, popmail): Use subroutines in pop.c to do the real work.
(get_errmsg, multiline, getline, putline, pop_stat, pop_command)
(pop_init): Functions deleted.