build_vms.com: use existing curlbuild.h and parsing fix

This patch removes building curlbuild.h from the build_vms.com procedure
and uses the one in the daily or release tarball instead.

packages/vms/build_curlbuild_h.com is obsolete with this change.

Accessing the library module name "tool_main" needs different handling
when the optional extended parsing is enabled.

Tested on IA64/VMS 8.4 and VAX/VMS 7.3