Consider safer injection method

Rather than using ` __attribute__((constructor))`, which is called early in the program's initialization sequence (before the call to _main_), make the library be a _readline(3)_ replacement, and call `dlsym(RTLD_NEXT, "readline")` after injecting the AI query behavior.

This will avoid the problem where with GNU awk _ai-cli-lib_ calls _malloc(3)_ before the awk code has called the required _pma_init(3)_, thus leading to a fatal _malloc(3)_ failure. This proposal was inspired by Terence Kelly's suggestion to avoid calling constructor-like functions before _main_.