Multiple alloca definitions in different files

We need to move the definition for alloca() into a separate h-file and include it from several places, as already done for ES editions. Because now it is included every time in a new way in different plugins, and with its own specifics in service_encryption, my_global, my_sys... If we leave it as it is - then some kind of desynchronization will be occurred from time to time - that in one of the operating systems like FreeBSD it will require us to do it differently, f.e. under new #ifdefs - but we may not correct one of plugins in this regard, and then something is not going as it should.