Fix handling of EBCDIC string fields.

Fix handling of EBCDIC string fields.

Have a routine that takes a 256-element translation table and uses it to
map various flavors of EBCDIC to Unicode.  Have separate translation
tables for "common" EBCDIC (everything that's the same in all EBCDIC
code pages that include the original EBCDIC characters) and EBCDIC code
page 037.  Add ENC_EBCDIC_CP037 for code page 037.

Change-Id: Ia882b3c0abef9e30eb54cd47396e6fa0d6342044
Reviewed-on: https://code.wireshark.org/review/19212
Reviewed-by: Guy Harris <guy@alum.mit.edu>