Handle munmap() and add support for tracing JNI (native) calls.

The munmap() kernel calls are traced but the tracing code wasn't doing
anything with them.  This caused the number of mapped regions in a process
to grow large in some cases and also caused symbol lookup errors in some
rare cases.  This change also adds support for new trace record types
for supporting JNI (native) calls from Java into native code. This helps
with constructing a more accurate call stack.