Unify startup and option-parsing warnings

Various warnings are emitted during startup and option-parsing using
fprintf_unfiltered.  One warning is prefixed with the command name,
the others are not.  This commit replaces these hardwired warnings
with calls to warning.  It also sets warning_pre_print to prefix all
warnings with the command name until option parsing is complete.

gdb/ChangeLog:

	* main.c (captured_main): Use warning during startup.
	Prefix startup warning messages with command name.