Refactor: libcrmcommon: separate IPC code into multiple files

A significant amount of new IPC code will be added soon, so avoid the file
sizes becoming ridiculous.

Before:

210 include/crm/common/ipc_internal.h

1846 lib/common/ipc.c
2056 total

After:

215 include/crm/common/ipc_internal.h
755 lib/common/ipc_client.c
103 lib/common/ipc_common.c
903 lib/common/ipc_server.c
146 lib/common/messages.c

2122 total

Rename newly exposed symbols per current style guidelines.