mock-server: make socket reading a bit more resilient.
read-prefs: add mongoc_read_prefs_add_tag() helper.

Just a simple helper to allow building a tag at a time with read
preferences.
mock-server: make reply_simple() usable from tests.
tests: add test for $readPreference in queries to mongos.
abi: fix symbol name.
collection: add _id in mongoc_collection_insert().
write-concern: hide visibility of private functions.
util: prefix private functions with _.
ssl: prefix private functions with _.
rpc: Prefix private functions with _.
read-prefs: seti visibility hidden on private functions.
queue: prefix private functions with _.
list: prefix private functions with _.
gridfs: prefix private functions with _.
gridfs-file-list: prefix private functions with _.
gridfs: prefix private functions with _.
gridfs-file: prefix private functions with _.
database: prefix private functions with _.
cursor: prefix private functions with _.
index: use default function instead of global extern.