Use lttng_read/lttng_write wrappers

This ensures we deal with EINTR and partial reads in the same way
everywhere.

This also fixes non-null-terminated string bug in
src/common/compat/compat-epoll.c and src/common/sessiond-comm/inet.c.

Also, lttng_pipe_read() and lttng_pipe_write() return values are changed
to simply match those of lttng_read() and lttng_write(). Their
implementation is replaced by the wrappers, except for locking and
checks.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>