Patched VMB.EXE to support boot block booting with the same logic that all of the ROM based VMB systems (MicroVAX I onward) handle interpreting the boot block.  Many alternate operating system installation media assumes that all VAX boot logic works this way.
Updated vmb_exe include file
Merge branch 'master' into VAX-11-BootBlock-Fixup
VAX 8600 Address map cleanup from Johnny Billquist
Merge branch 'master' into VAX-11-BootBlock-Fixup
VAX780, VAX739, VAX759, VAX8600: Merge branch 'VAX-11-BootBlock-Fixup' - Generic VMB.EXE for *nix VAX operating systems

 The native VMB.EXE program historically supported a Boot Block oriented
 boot if Bit 3 of the parameter register (R5) was set when VMB was invoked.
 This Boot Block boot operation reads sector 0 into memory and starts
 execution at offset 2 of the data block in memory.
 When portitions of VMB were migrated into ROM to support the earliest
 MicroVAX system (MicroVAX I) and all subsequent ROM based VMB versions
 the concept of Boot Block booting was extended in these ROM VMB
 implementations.  The change in boot block booting functionality included
 several features:
   1) If a normal boot attempt to a device failed (due to VMB not being
      able to locate a secondary bootstrap program), a boot block boot is
      automatically attempted.  If the Bit 3 of R5 was set, then the
      initial search for a secondary boot block was avoided and a boot
      block boot was immediately attempted.
   2) When performing a boot block boot, the sector 0 contents are examined
      and if these contents conform to the pattern defined for ROM based
      (PRA0) booting, the ROM format Offset, Size, and Starting address
      information is used directly by VMB to load a program into memory
      and control is transferred to that program.  If the contents of
      sector 0 do not fit the pattern required for ROM based booting, then
      the code in sector 0 is executed directly starting at offset 2,
      the same as was originally done with the non ROM based VMB versions.
      Note that this extended behavior allows sector 0 to contain very little
      information and quite possibly no actual boot code.
 Developers of alternate operating systems for VAX computers noticed the ROM
 based boot block behavior and changed their installation media AND the disk
 structures to only provide the minimal boot information required on the
 systems with VMB installed in ROM.

 Since, when this active development of these alternate operating systems for
 VAX computers was happening, the vast majority of development and new system
 deployments were to hardware which had ROM base VMB, no one noticed that
 older systems which booted with the non ROM based VMB could no longer boot
 from new install media or disks formatted with these operating systems.

 This patch addeds the ROM based VMB boot block boot functionality to the
 original dynamically loaded VMB.EXE used by the older systems to boot.

 The patch overwrites some VMB code which exists to support NVAX(1302) and
 Neon-V(1701) systems.  If simh simulators for these systems are ever built
 an alternate location must be found to accomodate this extended logic
Merge https://github.com/simh/simh