Introduce an "include" directive. It takes one argument, a filename
to be included into this one. This works the same way as #include
does in C; as far as the user is concerned, the included file is
inlined into the current one.

Since config(8) is no longer limited to working on one user-supplied
file, printing just a line number in an error message is not
sufficient. The new global variable yyfile represents the file
currently being parsed, and must be printed as well.

Reviewed by: imp
Obtained from: OpenBSD