GDB: testsuite: server: Don't return value from top-level (manual)

This patch manually changes "return -1" statements that weren't caught
by the sed command.

These return statements return at top level but inside block such as
save_vars and with_test_prefix.

I also fixed one case of top-level "return 0".

Approved-By: Tom Tromey <tom@tromey.com>