libwebsockets

spawn: in discrete env setting case do it readonly

OSX changed to blow a segfault on write to .rodata, exposing that
we're dropping a NUL in what can be .rodata to set the environment
manually.  We don't do this on Linux typically because we take the
code path where execvpe() is available to do the env for us.

Adapt the code to treat it as const, and underscore it by changing
its type to be const char ** in the info struct.