Clean this up a bit. Add a "tag_prefix" member. Move "useful_lsub" into

Clean this up a bit. Add a "tag_prefix" member. Move "useful_lsub" into

	* providers/imap/camel-imap-store.h: Clean this up a bit. Add a
	"tag_prefix" member. Move "useful_lsub" into capabilities.

	* providers/imap/camel-imap-store.c (camel_imap_store_init):
	Initialize the tag_prefix, based on a static variable.

	* providers/imap/camel-imap-command.c (camel_imap_command): Use
	the store's tag_prefix character rather than "A" at the start of
	the tag. Makes the verbose debug output easier to parse when
	connected to multiple IMAP servers. (Well, unless you're connected
	to more than 26 servers...)