Add scalloc() and use it in src/processes.c (#4014)

Add scalloc() and use it in src/processes.c (#4014)

* Add scalloc() wrapper similar to smalloc() to common utils

scalloc() wraps calloc() with exit on alloc failure,
similarly to what smalloc() does for malloc().

* Handle (Solaris-only) ps_read_process calloc fails by using scalloc

Everything else checks and handles calloc failures except this function.

As I cannot test Solaris specific code, I've just replaced calloc with
scalloc, which gracefully exits collectd with error message on alloc
failures (instead of corrupting memory / crashing, as would happen
with current code).