Fix memory leaks.
Check for termiox support at runtime.
Makefile.am: Add missing linux_termios.h.

If the file is not listed in SOURCES, it won't end up in the tarball.

Thanks to Dan Horák for the report.
Use new SP_ERR_SUPP error to indicate unsupported operations.
Remove dupliate/redundant return value documentation.
serialport.c: Add missing linux_termios.h #include.

This is safe to include (at the moment) on all non-win32 OSes, since it
doesn't contain anything OS-specific.