add libmodprobe api to query the number of modules loaded

In the short term this will be used to help implement support for
multiple kernel module directories but it may be useful in other
contexts as well.

Bug: 157645635
Change-Id: I15a252b6e9394292c8f3557ed65112c935830441
Merged-In: I15a252b6e9394292c8f3557ed65112c935830441