PR17232 variant #2: in runtime, let STP_EXIT nest within STP_START

PR17232 variant #2: in runtime, let STP_EXIT nest within STP_START

Uncommitted variant #1 consisted of using module refcounts in the
generated systemtap_module_init/exit function pair to ensure that an
uncleaned module cannot be unloaded.  That precluded cleanup via
rmmod(8), so a robust but inconvenient solution.

This variant #2 consists of a surgical fix, wherein an STP_EXIT
message comes in during an STP_START is used to set an atomic flag for
deferred _stp_cleanup_and_exit() handling.

A variant #3 is coming soon, using a protocol-wide command-message
mutex, like we did back before commit #262f7598.