cli: Simplify tracing with gdb.
egltrace: Improve spec of EGL context attributes.
egltrace: Retry without EGL_CONTEXT_OPENGL_DEBUG_BIT_KHR.

Seems to be causing problems with Mesa EGL drivers.
glxtrace: Use dlsym protection only on wrappers.

Similar in spirit to the Lawrence Love's pull request #203, but instead
of building multiple variants of glproc_gl.cpp simply move the
dlsym protection code into wrappers dir.