Feature: libcrmcommon: Add nvpair formatting functions.

These are used fairly extensively within crm_mon. They could either go
in as base format functions that everything has access to or as special
add-on messages that only crm_mon provides. The former seems more
generally useful.

Note that unlike all the other format functions, these return a string
instead of a void. My assumption is they will be used as part of other
functions and not do their own direct outputting.