gdb: create new is_thread_id helper function

This is a refactoring commit that splits the existing parse_thread_id
function into two parts, and then adds a new is_thread_id function.

The core of parse_thread_id is split into parse_thread_id_1, which is
responsible for actually parsing a thread-id.  Then parse_thread_id is
responsible for taking a parsed thread-id and validating that it
references an actually existing inferior thread.

The new is_thread_id function also uses parse_thread_id_1, but doesn't
actually check that the inferior thread exists, instead, this new
function simply checks that a string looks like a thread-id.

This commit does not add a use for is_thread_id, this will be added in
the next commit.

This is a refactoring commit, there should be no user visible changes
after this commit.