voicemail: Fix various abuses of mkstemp

mkstemp() returns a unique filename, but appending an extension to that
filename does not guarantee uniqueness. Instead, use mkdtemp() and we
can put whatever extension we want on the files that we create inside
the directory.

In the case of app_minivm, we also now properly clean up any temporary
files that we create.

ASTERISK-20858 #close
Reported by: Walter Doekes

Change-Id: I30ad04f0e115f0b11693ff678ba5184d8b938e43