Rename g_file_open() to g_file_open_rw(), and add a new g_file_open_ro()
call that wraps the common g_file_open_ex(file, 1, 0, 0, 0) idiom. This
will make the file access mode more explicit in the code.
Change all calls to g_file_open() to the _ro() or _rw() variant as
appropriate, and replace g_file_open_ex(file, 1, 0, 0, 0) with the _ro()
call.
Lastly, add tests for the two new calls to test_os_calls.c (code
courteously provided by matt335672).
The semantics of this call allowed it to be called more than once when
parsing a message to restart a parse. This is not likely to be
useful in practice, and it also makes reading file descriptors
more complicated. Consequently this function has been removed and
replaced with with libipm_msg_in_get_msgno()
Normally the test runners run in 'forking mode'. When a lot of
messages are logged to stdout, the child process can still be writing
data when the parent writes 'ok' or 'not ok'. This can confuse
the tap-driver.sh process.
