Use prefix matching to detect system namespaces, not substring matching

There are places in the code where we do

if ( strstr(ns, ".system.") )

or

if ( ns.find(".system.") != string::npos )

to detect system namespaces. Such substring matching is incorrect, since it returns true for namespaces like "db.foo.system.bar", which are not usually considered system namespaces. Matching should be prefix-based, as is now done for writes, for example.

This is along the same lines as SERVER-4558 and SERVER-8814, which refer to specific instances of this.

The cases that seem to be present currently are:

$ ag 'strstr *\(.* *"\.system\."'

src/mongo/db/query/get_executor.cpp

571:            if (strstr(ns, ".system.")) {

$ ag '\.find *\( *"\.system\."'

src/mongo/db/query/get_executor.cpp

145:                          || (string::npos != ns.find(".system."))

src/mongo/db/ops/insert.cpp

188:        if ( coll.find( ".system." ) != string::npos ) {

src/mongo/s/commands_admin.cpp

526:                if ( ns.find( ".system." ) != string::npos ) {

This ticket is to request that these occurrences in the code be updated to use prefix matches, not substring matches. Specifically:

• src/mongo/db/query/get_executor.cpp:571 appears to affect updates.
• src/mongo/db/query/get_executor.cpp:145 affects the behaviour of --notablescan, in that table scans may erroneously be permitted on collections that happen to contain an embedded ".system.".
• src/mongo/db/ops/insert.cpp:188 should be dropped completely, since it is just for backwards compatibility and the earlier code block at line 168 better handles system namespace detection.
• s/commands_admin.cpp:526 affects the ability to shard collections that happen to contain an embedded ".system.".

Probably somewhere there should be single isSystemNamespace()/isSystemCollection() function(s) for doing this detection consistently.