merge: refactor and document email functions

 * move functions to mutt_address.c
 * rename rfc822.c to address.c
 * move rfc822_new_address to address.c
 * standardise the address functions
 * standardise the envelope functions
 * standardise the parameter functions
 * move address functions
 * move envelope function
 * move parameter functions
 * rename mutt_tags to tags
 * add comments
 * rename rfc822 constants
 * minor tidying/boolification
 * move statics first