lib: Improve error reporting from broken config files

lib: Improve error reporting from broken config files

* command.h: (config_from_file) Add variable to interface for line
      number reporting.
    * command.c: (config_from_file) Set & increment 'line_num' while parsing.
    * vty.c: (vty_read_file) Report parse errors in the correct order to
      stderr, with added line numbers.

Conflicts:
	lib/command.c
	lib/vty.c

Integrated in changes made to continue loading code even after failure
to read a line in properly as part of this cherry-pick