Add support for BNDRY=PAGE on GETMAIN requests

[abekornelis]

Currently the BNDRY parameter is ignored. Change this so that the default is DBLWD, and PAGE is also accepted.
Debugging and dump reading is a lot easier when getmained storage can be page aligned.

Solution:
Allocate a bit in the register interface to indicate a BNDRY=PAGE request
Change the java code in sz390 that handles the getmain request to check that bit in the interface and allocate storage accordingly.

[abekornelis]

The logic in svc_getmain in sz390.java is not really suited for handling page-aligned requests. It will take quite a bit of rewriting.
Or maybe we should just route page-aligned requests to a new routine that will do just that ??

I did change the GETMAIN macro. I.e. mac\GETMAIN.MAC. See attached file. All changed/added lines are marked with #539
GETMAIN.MAC.txt