bug#29105: 25.2; sqli postgres missing prompts with underscore

[Felipe Ochoa]

Tables with underscores in their names (e.g., as generated by Django)
have their prompts removed from the buffer by the process filter,
resulting in something like the following

psql (9.6.5, server 9.5.5)
You are now connected to database "app_model" as user "user".
SELECT ...

I'm not sure how exactly it's happening, but I think it's in
`sql-interactive-remove-continuation-prompt'. At any rate, the problem
can be fixed by changing the postgres `:prompt-regexp' and
`:prompt-cont-regexp' to "^\\(?:[[:word:]]\\|\\s_\\)*=[#>] " and
"^\\(?:[[:word:]]\\|\\s_\\)*[-(][#>] " respectively to allow symbols in
table names. If that seems too lax, we could also special-case
underscore, since I think it's the most common symbol used in table
names.

-Felipe

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