gixsql errors on fields outside of EXEC SQL (like `LINKAGE`)

[GitMensch]

While gixpp used with gix needs to have more information, gixsql doesn't; it seems reasonable to have a "mode" in the driver so additional checks and the full tree can be skipped depending on that.

The current implementation of with gixpp for gixsql (that normally don't includes any copybooks - and shouldn't), along with a code as follows

       LINKAGE SECTION.
           COPY  MYROOT.
          05 FILLER PIC X.
           COPY  MYDATA.

break the checks and lead to a "parser error" (which is actually not a parser error, but a syntax error and - #154 misses the location):

parse error: FILLER level should start from 01 or 66 or 77 or 88

If there's a "mode" applied in the driver then everything outside of EXEC SQL DECLARE can be skipped (= it doesn't matter if this is "partial" code like the one above, it doesn't matter which exotic USAGEs or TYPEDEFs are used - as long as the parser understands that). At least in "gixsql mode" the field tree does not necessarily need to be built - or can at least silently ignoring "errors" (in the field case possibly add "implied" artificial fields with expected level as parent).

Note: currently there's a follow-up issue after this message as it leaves the driver without a current field, workaround for that sigsegv in #152.

Workaround:

        LINKAGE SECTION.
+          01 GIXDUMMY.
+            05 FILLER PIC X.
            COPY  MYROOT.
           05 FILLER PIC X.
            COPY  MYDATA.

[GitMensch]

@mridoni while the sigsegv is solved now, you likely want to inspect this as that is likely to be a problem for several production code COBOL.