gen_guc_tables.pl: Validate required GUC fields before code generation

gen_guc_tables.pl: Validate required GUC fields before code generation

Previously, gen_guc_tables.pl would emit "Use of uninitialized value"
warnings if required fields were missing in guc_parameters.dat (for
example, when an integer or real GUC omitted the 'max' value).  The
resulting error messages were unclear and did not identify which GUC
entry was problematic.

Add explicit validation of required fields depending on the parameter
type, and fail with a clear and specific message such as:

    guc_parameters.dat:1909: error: entry "max_index_keys" of type "int" is missing required field "max"

No changes to generated guc_tables.c.

Author: Chao Li <lic@highgo.com>
Reviewed-by: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://www.postgresql.org/message-id/flat/CAEoWx2%3DoP4LgHi771_OKhPPUS7B-CTqCs%3D%3DuQcNXWrwBoAm5Vg%40mail.gmail.com