include file search order

When searching for a file specified on the .incld directive, directories are searched in the following order:

1. Current directory

2. Directories specified by using the include option

3. default directories

   1. Directory specified by environment variable ASMCOP, if it exists

   2. Directory specified by environment variable COP, if it exists

   3. Directory \COP

If the xdirectory option is specified, only the directories specified by the include option are used.

Any file name that specifies an explicit directory is checked for only in that directory. No other directories are searched.