merge: add path functions to mailbox API

 * add name and magic to MxOps
 * add MxOps:path_probe()
 * improve mutt_path_canon
 * stubs for mx_path_{canon,pretty}
 * add mutt_str_inline_replace()
 * delegate mx_path_canon()
 * factor Folder out of imap_pretty_mailbox
 * add mutt_path_abbr_folder
 * fill out mx_path_pretty()
 * improve mutt_path_parent
 * add MxOps:path_parent()
 * tidy mx
 * use mx_path_probe()
 * probe notmuch
 * probe nntp
 * probe pop
 * probe imap
 * probe maildir