- writing to a PTY master side blocks if there is not already a
blocked reader on the slave side, and select now reflects this;
- internally, TTY now uses a test based on "caller != NONE" rather
than "grant != GRANT_INVALID" to identify whether a call is
currently ongoing;
- "offset" fields have been removed as they equal the corresponding
"cum" fields;
- improved variable typing and function naming here and there;
- various other small fixes.