Escape string and blob results from dump more correctly #2091

dumpTables currently badly handles BLOB and TEXT data containing control
characters:

MySQL will interpret and unescape string literals e.g.\r will become
carriage return.
Postgres will not allow string literals to contain NUL nor will
SQLite so BLOBs will not dump correctly.
Schemas should not be set on the destination dump
MSSQL needs the N prefix to correctly ensure that UTF-8 data is
correctly transferred.

Signed-off-by: Andrew Thornton art27@cantab.net

I need to go through MSSQL and the other non-Gitea enabled DB types to check that blobs and control characters are being properly handled there too.

OK, so I think I've got this working now.

However, large blobs and large text fields will not be being handled correctly.

MySQL - this doesn't appear to be a problem. Even \0 works in BLOB literals. I guess we need to decide if we want to use escaped strings or char() technique I've proposed here.
Postgres - pgdump uses COPY and an escaped format of its own design
Oracle - maybe something like

  dbms_lob.createtemporary(buf, FALSE);
  dbms_lob.append(buf, HEXTORAW('0EC1D7FA6B411DA58149'));
  --...lots of hex data...
  dbms_lob.append(buf, HEXTORAW('0EC1D7FA6B411DA58149'));

but I dunno

Escape string and blob results from dump more correctly #2091

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