Log: libcrmcluster: improve peer process set/clear messages

... instead of using set_bit()/clear_bit(), which improves trace messages
like:

Bit 0x04000000 set by some_function:100

to:

Peer process flags 0x04000000 (processes) for some_peer set by some_function:100

This uses pcmkset_flags_as() and pcmkclear_flags_as() directly rather than
creating separate macros, because they are only used once, and the long-term
intention is to merge the only value actually of use (crm_proc_cpg) into enum
crm_node_flags and drop these.