Rebase and tweak rocallahan:rework-syscall-wrapping, to clean up the syscall buffer a bit.

This is mainly a code cleanup, not adding significant new functionality.  A few changes are packaged up

* normalize usage of "syscall wrapper" to "syscall buffer", including renames
* use C functions instead of macros in syscall_buffer code
* handle more buffer overflow cases
* provide a single always-untraced syscall point to let syscall_buffer C code decide whether or not to buffer syscalls.  This moves logic out of the seccomp-bpf filter.
* add a stub for the gdb query qGetTLSAddr
* remove some newly-dead code
* add more debug logging to the recorder
* fix small bug in unit test
Merge pull request #148 from cgjones/syscallbuf-cleanup

Rebase and tweak rocallahan:rework-syscall-wrapping, to clean up the syscall buffer a bit.