Introduce complete_nested_command_line

Introduce complete_nested_command_line

This adds a completion helper routine that makes it possible for a
command that takes another command as argument, such as "frame apply
all COMMAND" as "thread apply all COMMAND", to complete on COMMAND,
and have the completion machinery recurse and complete COMMAND as if
you tried to complete "(gdb) COMMAND".  I.e., we'll be able to
complete like this, for example:

 (gdb) thread apply all -[TAB]
 -c           -ascending   -q           -s
 (gdb) thread apply all -ascending frame apply all -[TAB]
 -c           -limit       -past-entry  -past-main   -q           -s
 (gdb) thread apply all -ascending frame apply all -past-main print -[TAB]
 -address         -elements        -pretty          -symbol
 -array           -null-stop       -repeats         -union
 -array-indexes   -object          -static-members  -vtbl
 (gdb) thread apply all -ascending frame apply all -past-main print glo[TAB]
 global1         global2

Above, the completer function understands that "thread apply all" is a
command, and then parses "-ascending" successfully and understand that
the rest of the string is "thread apply all"'s operand.  And then, the
process repeats for the "frame apply" command, and on and on.

gdb/ChangeLog:
2019-06-13  Pedro Alves  <palves@redhat.com>

	* completer.c (complete_nested_command_line): New.
	(gdb_completion_word_break_characters_throw): Add assertion.
	* completer.h (complete_nested_command_line): Declare.