[PATCH] gsm0710muxd: Implement write buffering for the ptys

[Paul Fertser]

Hi,

Recently a new problem with gsm0710muxd was discovered. It appears
when there're lots of data from the modem and the receiving part of
the pty (frameworkd or pppd) for some reason is slow to read the
data. Reported by quickdev on IRC. Investigation showed that it's
because of silent overflow of the in-kernel pty buffer (4096 bytes).

The patch was tested and confirmed solving the issues by quickdev (who
had problems reading his large phonebook) and quatrox (who was unable
to receive large files via GPRS).

I have no experience with glib, but i performed some tests with a
simple application that showed that this kind of usage of glib buffers
is correct.