Zabbix

...G...... [ZBXNEXT-494] improvements and fixes for issues found in code review

Fixes:
- in procstat_alloc() we have to take into account the passed size when
  checking whether there is enough space in dynamic shared memory segment; 
- taking into account alignment when calculating how much space is needed for
  the initial shared memory allocation in procstat_add();
- calling exit() from procstat_alloc() in case there is not enough space in
  dynamic shared memory segment (since callers reserve space before allocation).

Coding improvements:
- introduced new macros to improve readibility of the code;
- split zbx_procstat_collect() into several shorter functions for improved
  readibility and debugging.